Condor uses clone in a way that causes GDB grief.  Running a Condor daemon under GDB will likely fail with errors similar to:

{code}
warning: Can't attach LWP -1: No such process
../../gdb/linux-thread-db.c:389: internal-error: thread_get_info_callback: Assertion `inout->thread_info != NULL' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
Quit this debugging session? (y or n)
{endcode}

The solution is to tell Condor to not to use clone, but to fall back on the fork code path.  In your Condor configuration file, use:

{code}
USE_CLONE_TO_CREATE_PROCESSES = false
{endcode}

(This page primarily exists as search bait, so that others hitting this error can quickly find the solution.)