[lordnacho]

Hard to pin down one in particular:

- Anything where you're looking for a race condition. It tends to be hard to reproduce, and instrumentation can make it go away entirely, leaving you with a need to conjecture about what might be happening. Quite satisfying when you find it, but again because it's rare you don't know if you're really solved it.

- Built a cross-platform, cross-language messaging system for trading. Combined UDP and TCP, had detection of downed servers. A lot of fiddling with network stuff, performance optimization on all platforms, both VM and native.