<rant>
My co-programmer at this consulting job has, I have discovered, no clue how to merge his changes to the code into the main tree. As such, every time he's integrated his changes, he's been clobbering one or two of the changes I'd made in the previous version. I only discovered this was happening today, unfortunately.

When I change the way the code works, I write a test. The test will fail if the old behaviour re-emerges. Unfortunately, this is no defense against someone who removes my changes from the thing that was being tested, and deletes the corresponding test, in a ham-handed attempt at getting his own code into the repository.

The paranoia is now: "What else has been deleted, that I don't know about because the thing I put there specifically to make sure it wasn't deleted was probably deleted too?"

Augh.
</rant>