Problem still seems to be present after 6 months. I'm running Feisty 

Linux acer-jn-u 2.6.20-16-generic #2 SMP Sun Sep 23 19:50:39 UTC 2007 i686 GNU/Linux

Removed skencil and reinstalled with no joy. Here's the dump. 

john@acer-jn-u:~$ skencil
*** glibc detected *** /usr/bin/python: free(): invalid pointer: 0xb7da6e18 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6[0xb7e4e7cd]
/lib/tls/i686/cmov/libc.so.6(cfree+0x90)[0xb7e51e30]
/usr/lib/skencil/Sketch/../Lib/paxmodule.so[0xb7ad5984]
/usr/bin/python[0x8110dca]
/usr/bin/python(PyEval_EvalFrameEx+0x6150)[0x80c8ec0]
/usr/bin/python(PyEval_EvalFrameEx+0x6135)[0x80c8ea5]
/usr/bin/python(PyEval_EvalFrameEx+0x6135)[0x80c8ea5]
/usr/bin/python(PyEval_EvalCodeEx+0x775)[0x80c9ca5]
/usr/bin/python(PyEval_EvalFrameEx+0x53f9)[0x80c8169]
/usr/bin/python(PyEval_EvalCodeEx+0x775)[0x80c9ca5]
/usr/bin/python[0x8111ff6]
/usr/bin/python(PyObject_Call+0x27)[0x805c787]
/usr/bin/python[0x80628e4]
/usr/bin/python(PyObject_Call+0x27)[0x805c787]
/usr/bin/python(PyEval_EvalFrameEx+0x2031)[0x80c4da1]
/usr/bin/python(PyEval_EvalCodeEx+0x775)[0x80c9ca5]
/usr/bin/python[0x8111ff6]
/usr/bin/python(PyObject_Call+0x27)[0x805c787]
/usr/bin/python[0x80628e4]
/usr/bin/python(PyObject_Call+0x27)[0x805c787]
/usr/bin/python(PyEval_CallObjectWithKeywords+0x6c)[0x80c1ebc]
/usr/bin/python(PyInstance_New+0x71)[0x8066031]
/usr/bin/python(PyObject_Call+0x27)[0x805c787]
/usr/bin/python(PyEval_EvalFrameEx+0x2031)[0x80c4da1]
/usr/bin/python(PyEval_EvalFrameEx+0x6135)[0x80c8ea5]
/usr/bin/python(PyEval_EvalCodeEx+0x775)[0x80c9ca5]
/usr/bin/python(PyEval_EvalCode+0x57)[0x80c9d17]
/usr/bin/python(PyRun_FileExFlags+0xf8)[0x80e9118]
/usr/bin/python(PyRun_SimpleFileExFlags+0x187)[0x80e93a7]
/usr/bin/python(Py_Main+0x9c0)[0x8059330]
/usr/bin/python(main+0x22)[0x8058862]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xdc)[0xb7dfcebc]
/usr/bin/python[0x80587b1]
======= Memory map: ========
08048000-0813f000 r-xp 00000000 08:03 1495763    /usr/bin/python2.5
0813f000-08164000 rw-p 000f6000 08:03 1495763    /usr/bin/python2.5
08164000-08497000 rw-p 08164000 00:00 0          [heap]
b6e00000-b6e21000 rw-p b6e00000 00:00 0 
b6e21000-b6f00000 ---p b6e21000 00:00 0 
b6f50000-b6f5b000 r-xp 00000000 08:03 325170     /lib/libgcc_s.so.1
b6f5b000-b6f5c000 rw-p 0000a000 08:03 325170     /lib/libgcc_s.so.1
b6f6e000-b6f70000 r-xp 00000000 08:03 1660026    /usr/lib/python2.5/lib-dynload/fcntl.so
b6f70000-b6f71000 rw-p 00002000 08:03 1660026    /usr/lib/python2.5/lib-dynload/fcntl.so
b6f71000-b6f73000 r-xp 00000000 08:03 1660020    /usr/lib/python2.5/lib-dynload/_random.so
b6f73000-b6f74000 rw-p 00002000 08:03 1660020    /usr/lib/python2.5/lib-dynload/_random.so
b6f74000-b6f78000 r-xp 00000000 08:03 1660025    /usr/lib/python2.5/lib-dynload/binascii.so
b6f78000-b6f79000 rw-p 00003000 08:03 1660025    /usr/lib/python2.5/lib-dynload/binascii.so
b6f79000-b6f7a000 ---p b6f79000 00:00 0 
b6f7a000-b777a000 rw-p b6f7a000 00:00 0 
b777a000-b7780000 r-xp 00000000 08:03 54499      /usr/lib/skencil/Lib/paxtkinter.so
b7780000-b7781000 rw-p 00005000 08:03 54499      /usr/lib/skencil/Lib/paxtkinter.so
b7781000-b7784000 r-xp 00000000 08:03 54246      /usr/lib/skencil/Sketch/Modules/pstokenize.so
b7784000-b7785000 rw-p 00002000 08:03 54246      /usr/lib/skencil/Sketch/Modules/pstokenize.so
b7785000-b7786000 r-xp 00000000 08:03 54244      /usr/lib/skencil/Sketch/Modules/_type1module.so
b7786000-b7787000 rw-p 00000000 08:03 54244      /usr/lib/skencil/Sketch/Modules/_type1module.so
b7787000-b778a000 r-xp 00000000 08:03 1660023    /usr/lib/python2.5/lib-dynload/cStringIO.so
b778a000-b778b000 rw-p 00003000 08:03 1660023    /usr/lib/python2.5/lib-dynload/cStringIO.so
b778b000-b7791000 r-xp 00000000 08:03 54502      /usr/lib/skencil/Lib/streamfilter.so
b7791000-b7792000 rw-p 00005000 08:03 54502      /usr/lib/skencil/Lib/streamfilter.so
b7792000-b78b6000 rw-p b7792000 00:00 0 
b78b6000-b78c9000 r-xp 00000000 08:03 1499261    /usr/lib/libz.so.1.2.3
b78c9000-b78ca000 rw-p 00012000 08:03 1499261    /usr/lib/libz.so.1.2.3
b78ca000-b78e8000 r-xp 00000000 08:03 1497526    /usr/lib/libjpeg.so.62.0.0
b78e8000-b78e9000 rw-p 0001d000 08:03 1497526    /usr/lib/libjpeg.so.62.0.0
b78e9000-b791b000 r-xp 00000000 08:03 1794275    /usr/lib/python2.5/site-packages/PIL/_imaging.so
b791b000-b791e000 rw-p 00032000 08:03 1794275    /usr/lib/python2.5/site-packages/PIL/_imaging.so
b791e000-b7926000 rw-p b791e000 00:00 0 
b7926000-b7933000 r-xp 00000000 08:03 1497054    /usr/lib/libXext.so.6.4.0
b7933000-b7934000 rw-p 0000d000 08:03 1497054    /usr/lib/libXext.so.6.4.0
b7934000-b79e3000 r-xp 00000000 08:03 1500875    /usr/lib/libtcl8.4.so.0
b79e3000-b79e9000 rw-p 000af000 08:03 1500875    /usr/lib/libtcl8.4.so.0
b79e9000-b7abc000 r-xp 00000000 08:03 1499033    /usr/lib/libtk8.4.so.0
b7abc000-b7ac8000 rw-p 000d3000 08:03 1499033    /usr/lib/libtk8.4.so.0
b7ac8000-b7ad9000 r-xp 00000000 08:03 54498      /usr/lib/skencil/Lib/paxmodule.so
b7ad9000-b7adb000 rw-p 00010000 08:03 54498      /usr/lib/skencil/Lib/paxmodule.so
b7adb000-b7adf000 r-xp 00000000 08:03 1496465    /usr/lib/libXdmcp.so.6.0.0
b7adf000-b7ae0000 rw-p 00003000 08:03 1496465    /usr/lib/libXdmcp.so.6.0.0
b7ae0000-b7ae2000 r-xp 00000000 08:03 1496018    /usr/lib/libXau.so.6.0.0
b7ae2000-b7ae3000 rw-p 00001000 08:03 1496018    /usr/lib/libXau.so.6.0.0
b7ae3000-b7bd0000 r-xp 00000000 08:03 1496630    /usr/lib/libX11.so.6.2.0
b7bd0000-b7bd4000 rw-p 000ed000 08:03 1496630    /usr/lib/libX11.so.6.2.0
b7bd5000-b7bd8000 r-xp 00000000 08:03 1660034    /usr/lib/python2.5/lib-dynload/time.so
b7bd8000-b7bda000 rw-p 00002000 08:03 1660034    /usr/lib/python2.5/lib-dynload/time.so
b7bda000-b7bdd000 r-xp 00000000 08:03 1660029    /usr/lib/python2.5/lib-dynload/math.so
b7bdd000-b7bde000 rw-p 00002000 08:03 1660029    /usr/lib/python2.5/lib-dynload/math.so
b7bde000-b7be4000 r-xp 00000000 08:03 1660024    /usr/lib/python2.5/lib-dynload/array.so
b7be4000-b7be6000 rw-p 00006000 08:03 1660024    /usr/lib/python2.5/lib-dynload/array.so
b7be6000-b7bfd000 r-xp 00000000 08:03 54243      /usr/lib/skencil/Sketch/Modules/_sketchmodule.so
b7bfd000-b7bff000 rw-p 00016000 08:03 54243      /usr/lib/skencil/Sketch/Modules/_sketchmodule.so
b7bff000-b7c00000 r--p 00000000 08:03 1561708    /usr/lib/locale/en_US.utf8/LC_NUMERIC
b7c00000-b7c01000 r--p 00000000 08:03 1220307    /usr/lib/locale/en_US.utf8/LC_TIME
b7c01000-b7cd8000 r--p 00000000 08:03 1561702    /usr/lib/locale/en_US.utf8/LC_COLLATE
b7cd8000-b7cd9000 r--p 00000000 08:03 1220495    /usr/lib/locale/en_US.utf8/LC_MONETARY
b7cd9000-b7cda000 r--p 00000000 08:03 1561751    /usr/lib/locale/en_US.utf8/LC_MESSAGES/SYS_LC_MESSAGES
b7cda000-b7cdb000 r--p 00000000 08:03 1561839    /usr/lib/locale/en_US.utf8/LC_PAPER
b7cdb000-b7d1c000 rw-p b7cdb000 00:00 0 
b7d1c000-b7d20000 r-xp 00000000 08:03 1660032    /usr/lib/python2.5/lib-dynload/strop.so
b7d20000-b7d22000 rw-p 00003000 08:03 1660032    /usr/lib/python2.5/lib-dynload/strop.so
b7d22000-b7d29000 r--s 00000000 08:03 1499170    /usr/lib/gconv/gconv-modules.cache
b7d29000-b7d64000 r--p 00000000 08:03 1040393    /usr/lib/locale/en_US.utf8/LC_CTYPE
b7d64000-b7de7000 rw-p b7d64000 00:00 0 
b7de7000-b7f22000 r-xp 00000000 08:03 325370     /lib/tls/i686/cmov/libc-2.5.so
b7f22000-b7f23000 r--p 0013b000 08:03 325370     /lib/tls/i686/cmov/libc-2.5.so
b7f23000-b7f25000 rw-p 0013c000 08:03 325370     /lib/tls/i686/cmov/libc-2.5.so
b7f25000-b7f28000 rw-p b7f25000 00:00 0 
b7f28000-b7f4d000 r-xp 00000000 08:03 325374     /lib/tls/i686/cmov/libm-2.5.so
b7f4d000-b7f4f000 rw-p 00024000 08:03 325374     /lib/tls/i686/cmov/libm-2.5.so
b7f4f000-b7f51000 r-xp 00000000 08:03 325389     /lib/tls/i686/cmov/libutil-2.5.so
b7f51000-b7f53000 rw-p 00001000 08:03 325389     /lib/tls/i686/cmov/libutil-2.5.so
b7f53000-b7f54000 rw-p b7f53000 00:00 0 
b7f54000-b7f56000 r-xp 00000000 08:03 325373     /lib/tls/i686/cmov/libdl-2.5.so
b7f56000-b7f58000 rw-p 00001000 08:03 325373     /lib/tls/i686/cmov/libdl-2.5.so
b7f58000-b7f6b000 r-xp 00000000 08:03 325384     /lib/tls/i686/cmov/libpthread-2.5.so
b7f6b000-b7f6d000 rw-p 00013000 08:03 325384     /lib/tls/i686/cmov/libpthread-2.5.so
b7f6d000-b7f6f000 rw-p b7f6d000 00:00 0 
b7f6f000-b7f70000 r--p 00000000 08:03 1221787    /usr/lib/locale/en_US.utf8/LC_NAME
b7f70000-b7f71000 r--p 00000000 08:03 1221788    /usr/lib/locale/en_US.utf8/LC_ADDRESS
b7f71000-b7f72000 r--p 00000000 08:03 1222460    /usr/lib/locale/en_US.utf8/LC_TELEPHONE
b7f72000-b7f73000 r--p 00000000 08:03 1222461    /usr/lib/locale/en_US.utf8/LC_MEASUREMENT
b7f73000-b7f74000 r--p 00000000 08:03 1222462    /usr/lib/locale/en_US.utf8/LC_IDENTIFICATION
b7f74000-b7f79000 r-xp 00000000 08:03 1660030    /usr/lib/python2.5/lib-dynload/operator.so
b7f79000-b7f7a000 rw-p 00005000 08:03 1660030    /usr/lib/python2.5/lib-dynload/operator.so
b7f7a000-b7f7d000 r-xp 00000000 08:03 1659980    /usr/lib/python2.5/lib-dynload/_locale.so
b7f7d000-b7f7e000 rw-p 00003000 08:03 1659980    /usr/lib/python2.5/lib-dynload/_locale.so
b7f7e000-b7f80000 r-xp 00000000 08:03 54500      /usr/lib/skencil/Lib/intl.so
b7f80000-b7f81000 rw-p 00001000 08:03 54500      /usr/lib/skencil/Lib/intl.so
b7f81000-b7f83000 rw-p b7f81000 00:00 0 
b7f83000-b7f9c000 r-xp 00000000 08:03 325584     /lib/ld-2.5.so
b7f9c000-b7f9e000 rw-p 00019000 08:03 325584     /lib/ld-2.5.so
bfa4c000-bfa67000 rw-p bfa4c000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]
Aborted (core dumped)
john@acer-jn-u:~$