Some applications don't themselves link against libpthread, but then load plugins which do link against libpthread. This means internally switching from single-threading to multi-threading, which is only supported since libc0.3 2.19-16~2. Previously, it would result in errors such as:

./pthread/../sysdeps/generic/pt-mutex-timedlock.c:70: __pthread_mutex_timedlock_internal: Assertion `__pthread_threads' failed.

This could be worked around by making the application link against libpthread (i.e. not only the plugin, but also the main binary), or without recompiling by explicitly pre-loading libpthread, for example:

$ LD_PRELOAD=/lib/i386-gnu/libpthread.so.0.3 [application]

But it should now be gone, simply upgrade libc0.3.