There is a whole taxonomy of bugs that I only just found courtesy of Steve Hawley.
How did I not know about this? The term Heisenbug is two decades old!
I particular like Schroedinbug: "A schroedinbug is a bug that manifests only after someone reading source code or using the program in an unusual way notices that it never should have worked in the first place, at which point the program promptly stops working for everybody until fixed." - very Douglas Adams.