I can still produce this bug on Debian stable. It requires me to run a process that does a lot of random io in a a large file, such that some swap is consumed.

I also had an instance of X crashing as a result:

[ 84441.029] BUG: triggered 'if (priv->num_active_touches > priv->num_slots)'
[ 84441.029] BUG: ../../src/synaptics.c:3122 in UpdateTouchState()
[ 84441.029]
[ 84441.029] Backtrace:
[ 84441.029] 0: /usr/bin/X (xorg_backtrace+0x36) [0x7f9db58fbd16]
[ 84441.029] 1: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x3067) [0x7f9dafb4b067]
[ 84441.029] 2: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x4733) [0x7f9dafb4c733]
[ 84441.029] 3: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x7164) [0x7f9dafb4f164]
[ 84441.029] 4: /usr/bin/X (0x7f9db577d000+0x8d947) [0x7f9db580a947]
[ 84441.029] 5: /usr/bin/X (0x7f9db577d000+0xb1c18) [0x7f9db582ec18]
[ 84441.029] 6: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f9db4aa5000+0xf0a0) [0x7f9db4ab40a0]
[ 84441.029] 7: /lib/x86_64-linux-gnu/libc.so.6 (__select+0x13) [0x7f9db3881293]
[ 84441.029] 8: /usr/bin/X (WaitForSomething+0x190) [0x7f9db58f9150]
[ 84441.029] 9: /usr/bin/X (0x7f9db577d000+0x52bb1) [0x7f9db57cfbb1]
[ 84441.029] 10: /usr/bin/X (0x7f9db577d000+0x41ec5) [0x7f9db57beec5]
[ 84441.029] 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xfd) [0x7f9db37c9ead]
[ 84441.029] 12: /usr/bin/X (0x7f9db577d000+0x4219d) [0x7f9db57bf19d]
[ 84441.029]
[ 84441.115] BUG: triggered 'if (priv->num_active_touches > priv->num_slots)'
[ 84441.115] BUG: ../../src/synaptics.c:3122 in UpdateTouchState()
[ 84441.115]
[ 84441.115] Backtrace:
[ 84441.115] 0: /usr/bin/X (xorg_backtrace+0x36) [0x7f9db58fbd16]
[ 84441.115] 1: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x3067) [0x7f9dafb4b067]
[ 84441.115] 2: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x4733) [0x7f9dafb4c733]
[ 84441.115] 3: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x7164) [0x7f9dafb4f164]
[ 84441.115] 4: /usr/bin/X (0x7f9db577d000+0x8d947) [0x7f9db580a947]
[ 84441.115] 5: /usr/bin/X (0x7f9db577d000+0xb1c18) [0x7f9db582ec18]
[ 84441.115] 6: /lib/x86_64-linux-gnu/libpthread.so.0 (0x7f9db4aa5000+0xf0a0) [0x7f9db4ab40a0]
[ 84441.115] 7: /lib/x86_64-linux-gnu/libc.so.6 (__select+0x13) [0x7f9db3881293]
[ 84441.115] 8: /usr/bin/X (WaitForSomething+0x190) [0x7f9db58f9150]
[ 84441.115] 9: /usr/bin/X (0x7f9db577d000+0x52bb1) [0x7f9db57cfbb1]
[ 84441.115] 10: /usr/bin/X (0x7f9db577d000+0x41ec5) [0x7f9db57beec5]
[ 84441.115] 11: /lib/x86_64-linux-gnu/libc.so.6 (__libc_start_main+0xfd) [0x7f9db37c9ead]
[ 84441.115] 12: /usr/bin/X (0x7f9db577d000+0x4219d) [0x7f9db57bf19d]
[ 84441.115]
[ 84442.200] BUG: triggered 'if (priv->num_active_touches > priv->num_slots)'
[ 84442.200] BUG: ../../src/synaptics.c:3122 in UpdateTouchState()
[ 84442.200]
[ 84442.200] Backtrace:
[ 84442.200] 0: /usr/bin/X (xorg_backtrace+0x36) [0x7f9db58fbd16]
[ 84442.200] 1: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x3067) [0x7f9dafb4b067]
[ 84442.200] 2: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x4733) [0x7f9dafb4c733]
[ 84442.200] 3: /usr/lib/xorg/modules/input/synaptics_drv.so (0x7f9dafb48000+0x7164) [0x7f9dafb4f164]
[ 84442.200] 4: /usr/bin/X (0x7f9db577d000+0x8d947) [0x7f9db580a947]
[ 84442.200] 5: /usr/bin/X (0x7f9db577d000+0xb1c18) [0x7f9db582ec18]
[ 84442.200]