Yes, this has been my experience too! Another tool in the toolbox is property / fuzz testing. Especially for data structures, and anything that looks like a state machine. My typical setup is this:

1. Make a list of invariants. (Eg if Foo is set, bar + zot must be less than 10)

2. Make a check() function which validates all the invariants you can think of. It’s ok if this function is slow.

3. Make a function which takes in a random seed. It initializes your object and then, in a loop, calls random mutation functions (using a seeded RNG) and then calls check(). 100 iterations is usually a good number.

4. Call this in an outer loop, trying lots of seeds.

5. If anything fails, print out the failing seed number and crash. This provides a reproducible test so you can go in and figure out what went wrong.

If I had a penny for every bug I’ve found doing this, I’d be a rich man. It’s a wildly effective technique.