Up till now I could only come up with small tests that succeeded and large scale tests that failed.
What I needed was a small test that failed. I finally have one. And even better, it actually fails in the debugger :-)
It's not so easy to come up with a small failing test because to do that you have to narrow down which part of the code is failing. Which is half the challenge, the other half is to figure out why it's failing.
At least now I feel like I have the beast cornered and it's only a matter of time before I kill it.
The test is simple enough that I look at it and think "this can't fail". But it is failing, so obviously I'm missing something. I just hope it's not something too obvious in hindsight because then I'll feel really stupid when I find it.