One nightmarish project that was doomed from the beginning, had me as the sole developer. I could hardly sleep when we began testing on a separate test system, but with (nearly) all the config stored in shared memory and copied from the production system, I dreaded, half awake, that the production server data base connection was still configured in the test system and that it was shooting all it's test data repeatedly to prod.

Finally drove to company in middle of the night at 4 o'clock. Checked everything was OK, tried to sleep 3 hours before the start of the work day.

This system also had the most hideous memory corruption in some shared memory that was used across several processes and should have been thoroughly protected by a mutex, but somehow, sometimes this crucial map, that was used to speed up the access to all the customer data just contained garbage.

Still haunts me to that day. (Like xkcd's unresolved tension of a non-matching parenthesis - an unresolved bug.