I've seen this behavior in KeepassX 0.4.3 on Ubuntu 10.04 starting earlier this year.  It's 100% reproducible and happens only on save.  The crash message is:

*** glibc detected *** keepassx: free(): invalid next size (normal): 0x08baa418 ***
======= Backtrace: =========
/lib/tls/i686/cmov/libc.so.6(+0x6b161)[0x93d161]
/lib/tls/i686/cmov/libc.so.6(+0x6c9b8)[0x93e9b8]
/lib/tls/i686/cmov/libc.so.6(cfree+0x6d)[0x941a9d]
/usr/lib/libstdc++.so.6(_ZdlPv+0x21)[0x223741]
/usr/lib/libstdc++.so.6(_ZdaPv+0x1d)[0x22379d]
keepassx[0x80974d7]
keepassx[0x8074077]
keepassx[0x8111c4a]
/usr/lib/libQtCore.so.4(_ZN11QMetaObject8metacallEP7QObjectNS_4CallEiPPv+0x3a)[0x456c9a]
/usr/lib/libQtCore.so.4(_ZN11QMetaObject8activateEP7QObjectPKS_iPPv+0x2d5)[0x4653d5]
/usr/lib/libQtGui.so.4(_ZN7QAction9triggeredEb+0x49)[0xf88bd9]
/usr/lib/libQtGui.so.4(_ZN7QAction8activateENS_11ActionEventE+0x7c)[0xf8adcc]
/usr/lib/libQtGui.so.4(_ZN7QAction5eventEP6QEvent+0xe8)[0xf8c698]
/usr/lib/libQtGui.so.4(_ZN19QApplicationPrivate13notify_helperEP7QObjectP6QEvent+0xac)[0xf8f4dc]
/usr/lib/libQtGui.so.4(_ZN12QApplication6notifyEP7QObjectP6QEvent+0x17e)[0xf9605e]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication14notifyInternalEP7QObjectP6QEvent+0x7b)[0x451a3b]
/usr/lib/libQtGui.so.4(+0x17734d)[0xfd134d]
/usr/lib/libQtGui.so.4(+0x17915e)[0xfd315e]
/usr/lib/libQtGui.so.4(_ZN12QApplication6notifyEP7QObjectP6QEvent+0x1f63)[0xf97e43]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication14notifyInternalEP7QObjectP6QEvent+0x7b)[0x451a3b]
/usr/lib/libQtGui.so.4(+0x1362be)[0xf902be]
/usr/lib/libQtGui.so.4(+0x1efd90)[0x1049d90]
/usr/lib/libQtGui.so.4(+0x1f22f5)[0x104c2f5]
/usr/lib/libQtGui.so.4(_ZN12QApplication15x11ProcessEventEP7_XEvent+0x9b7)[0x101f757]
/usr/lib/libQtGui.so.4(+0x1f560a)[0x104f60a]
/lib/libglib-2.0.so.0(g_main_context_dispatch+0x1d5)[0x5a25e5]
/lib/libglib-2.0.so.0(+0x3f2d8)[0x5a62d8]
/lib/libglib-2.0.so.0(g_main_context_iteration+0x68)[0x5a64b8]
/usr/lib/libQtCore.so.4(_ZN20QEventDispatcherGlib13processEventsE6QFlagsIN10QEventLoop17ProcessEventsFlagEE+0x65)[0x47d5d5]
/usr/lib/libQtGui.so.4(+0x1f5135)[0x104f135]
/usr/lib/libQtCore.so.4(_ZN10QEventLoop13processEventsE6QFlagsINS_17ProcessEventsFlagEE+0x49)[0x450059]
/usr/lib/libQtCore.so.4(_ZN10QEventLoop4execE6QFlagsINS_17ProcessEventsFlagEE+0xfa)[0x4504aa]
/usr/lib/libQtCore.so.4(_ZN16QCoreApplication4execEv+0xaf)[0x45469f]
/usr/lib/libQtGui.so.4(_ZN12QApplication4execEv+0x27)[0xf8f577]
keepassx[0x806c35c]
/lib/tls/i686/cmov/libc.so.6(__libc_start_main+0xe6)[0x8e8bd6]
keepassx[0x805f031]
======= Memory map: ========
00110000-00128000 r-xp 00000000 fc:04 165499     /usr/lib/libxcb.so.1.1.0
00128000-00129000 r--p 00017000 fc:04 165499     /usr/lib/libxcb.so.1.1.0
00129000-0012a000 rw-p 00018000 fc:04 165499     /usr/lib/libxcb.so.1.1.0
0012a000-00138000 r-xp 00000000 fc:04 165766     /usr/lib/libXext.so.6.4.0
00138000-00139000 r--p 0000d000 fc:04 165766     /usr/lib/libXext.so.6.4.0
00139000-0013a000 rw-p 0000e000 fc:04 165766     /usr/lib/libXext.so.6.4.0
0013a000-0014f000 r-xp 00000000 fc:04 166509     /usr/lib/libaudio.so.2.4
0014f000-00150000 r--p 00015000 fc:04 166509     /usr/lib/libaudio.so.2.4
00150000-00151000 rw-p 00016000 fc:04 166509     /usr/lib/libaudio.so.2.4
00151000-00164000 r-xp 00000000 fc:04 135474     /lib/libz.so.1.2.3.3
00164000-00165000 r--p 00012000 fc:04 135474     /lib/libz.so.1.2.3.3
00165000-00166000 rw-p 00013000 fc:04 135474     /lib/libz.so.1.2.3.3
00167000-00168000 r-xp 00000000 00:00 0          [vdso]
00168000-00251000 r-xp 00000000 fc:04 131854     /usr/lib/libstdc++.so.6.0.13
00251000-00255000 r--p 000e9000 fc:04 131854     /usr/lib/libstdc++.so.6.0.13
00255000-00256000 rw-p 000ed000 fc:04 131854     /usr/lib/libstdc++.so.6.0.13
00256000-0025d000 rw-p 00000000 00:00 0 
0025d000-0028b000 r-xp 00000000 fc:04 167520     /usr/lib/libfontconfig.so.1.4.4
0028b000-0028c000 r--p 0002d000 fc:04 167520     /usr/lib/libfontconfig.so.1.4.4
0028c000-0028d000 rw-p 0002e000 fc:04 167520     /usr/lib/libfontconfig.so.1.4.4
0028d000-002b0000 r-xp 00000000 fc:04 137747     /lib/libpng12.so.0.42.0
002b0000-002b1000 r--p 00022000 fc:04 137747     /lib/libpng12.so.0.42.0
002b1000-002b2000 rw-p 00023000 fc:04 137747     /lib/libpng12.so.0.42.0
002b2000-002c7000 r-xp 00000000 fc:04 165772     /usr/lib/libICE.so.6.3.0
002c7000-002c8000 r--p 00014000 fc:04 165772     /usr/lib/libICE.so.6.3.0
002c8000-002c9000 rw-p 00015000 fc:04 165772     /usr/lib/libICE.so.6.3.0
002c9000-002cb000 rw-p 00000000 00:00 0 
002cc000-002e7000 r-xp 00000000 fc:04 146474     /lib/ld-2.11.1.so
002e7000-002e8000 r--p 0001a000 fc:04 146474     /lib/ld-2.11.1.so
002e8000-002e9000 rw-p 0001b000 fc:04 146474     /lib/ld-2.11.1.so
002e9000-0055f000 r-xp 00000000 fc:04 135339     /usr/lib/libQtCore.so.4.6.2
0055f000-00566000 r--p 00275000 fc:04 135339     /usr/lib/libQtCore.so.4.6.2
00566000-00567000 rw-p 0027c000 fc:04 135339     /usr/lib/libQtCore.so.4.6.2
00567000-0062f000 r-xp 00000000 fc:04 135118     /lib/libglib-2.0.so.0.2400.1
0062f000-00630000 r--p 000c7000 fc:04 135118     /lib/libglib-2.0.so.0.2400.1
00630000-00631000 rw-p 000c8000 fc:04 135118     /lib/libglib-2.0.so.0.2400.1
00631000-00635000 r-xp 00000000 fc:04 135121     /usr/lib/libgthread-2.0.so.0.2400.1
00635000-00636000 r--p 00003000 fc:04 135121     /usr/lib/libgthread-2.0.so.0.2400.1
00636000-00637000 rw-p 00004000 fc:04 135121     /usr/lib/libgthread-2.0.so.0.2400.1
00637000-0063b000 r-xp 00000000 fc:04 165497     /usr/lib/libXdmcp.so.6.0.0
0063b000-0063c000 r--p 00003000 fc:04 165497     /usr/lib/libXdmcp.so.6.0.0
0063c000-0063d000 rw-p 00004000 fc:04 165497     /usr/lib/libXdmcp.so.6.0.0
0063d000-00661000 r-xp 00000000 fc:04 165866     /lib/libexpat.so.1.5.2
00661000-00663000 r--p 00024000 fc:04 165866     /lib/libexpat.so.1.5.2
00663000-00664000 rw-p 00026000 fc:04 165866     /lib/libexpat.so.1.5.2
00664000-00666000 r-xp 00000000 fc:04 146221     /usr/lib/gconv/UTF-16.so
00666000-00667000 r--p 00001000 fc:04 146221     /usr/lib/gconv/UTF-16.so
00667000-00668000 rw-p 00002000 fc:04 146221     /usr/lib/gconv/UTF-16.so
00668000-00685000 r-xp 00000000 fc:04 148879     /lib/libgcc_s.so.1
00685000-00686000 r--p 0001c000 fc:04 148879     /lib/libgcc_s.so.1
00686000-00687000 rw-p 0001d000 fc:04 148879     /lib/libgcc_s.so.1
00687000-006c4000 r-xp 00000000 fc:04 135119     /usr/lib/libgobject-2.0.so.0.2400.1
006c4000-006c5000 r--p 0003c000 fc:04 135119     /usr/lib/libgobject-2.0.so.0.2400.1
006c5000-006c6000 rw-p 0003d000 fc:04 135119     /usr/lib/libgobject-2.0.so.0.2400.1
006c6000-006cc000 r-xp 00000000 fc:04 173618     /usr/lib/libXrandr.so.2.2.0
006cc000-006cd000 r--p 00005000 fc:04 173618     /usr/lib/libXrandr.so.2.2.0
006cd000-006ce000 rw-p 00006000 fc:04 173618     /usr/lib/libXrandr.so.2.2.0
006ce000-006d2000 r-xp 00000000 fc:04 165949     /usr/lib/libXfixes.so.3.1.0
006d2000-006d3000 r--p 00003000 fc:04 165949     /usr/lib/libXfixes.so.3.1.0
006d3000-006d4000 rw-p 00004000 fc:04 165949     /usr/lib/libXfixes.so.3.1.0
006d4000-006dc000 r-xp 00000000 fc:04 173600     /usr/lib/libXcursor.so.1.0.2
006dc000-006dd000 r--p 00007000 fc:04 173600     /usr/lib/libXcursor.so.1.0.2
006dd000-006de000 rw-p 00008000 fc:04 173600     /usr/lib/libXcursor.so.1.0.2
006de000-006df000 r-xp 00000000 fc:04 136072     /usr/lib/gtk-2.0/2.10.0/loaders/svg_loader.so
006df000-006e0000 r--p 00000000 fc:04 136072     /usr/lib/gtk-2.0/2.10.0/loaders/svg_loader.so
006e0000-006e1000 rw-p 00001000 fc:04 136072     /usr/lib/gtk-2.0/2.10.0/loaders/svg_loader.so
006e1000-006ed000 r-xp 00000000 fc:04 165961     /usr/lib/libXi.so.6.1.0
006ed000-006ee000 r--p 0000c000 fc:04 165961     /usr/lib/libXi.so.6.1.0
006ee000-006ef000 rw-p 0000d000 fc:04 165961     /usr/lib/libXi.so.6.1.0
006f2000-0080b000 r-xp 00000000 fc:04 165760     /usr/lib/libX11.so.6.3.0
0080b000-0080c000 r--p 00118000 fc:04 165760     /usr/lib/libX11.so.6.3.0
0080c000-0080e000 rw-p 00119000 fc:04 165760     /usr/lib/libX11.so.6.3.0
0080e000-0080f000 rw-p 00000000 00:00 0 
0080f000-00880000 r-xp 00000000 fc:04 133873     /usr/lib/libfreetype.so.6.3.22
00880000-00884000 r--p 00070000 fc:04 133873     /usr/lib/libfreetype.so.6.3.22
00884000-00885000 rw-p 00074000 fc:04 133873     /usr/lib/libfreetype.so.6.3.22
00885000-0088c000 r-xp 00000000 fc:04 146507     /lib/tls/i686/cmov/librt-2.11.1.so
0088c000-0088d000 r--p 00006000 fc:04 146507     /lib/tls/i686/cmov/librt-2.11.1.so
0088d000-0088e000 rw-p 00007000 fc:04 146507     /lib/tls/i686/cmov/librt-2.11.1.so
0088e000-00890000 r-xp 00000000 fc:04 173612     /usr/lib/libXinerama.so.1.0.0
00890000-00891000 r--p 00001000 fc:04 173612     /usr/lib/libXinerama.so.1.0.0Aborted

This behavior is tied to the database.  If I export to XML, create a new database and import the XML, it's fine.  Sorry, I can't include the problematic database.