Most common reasons for unstable bugs in C++?

Question

I am currently working on a large project, and I spend most of the time debugging. While debugging is a normal process, there are bugs, that are unstable, and these bugs are

Answer 1

Not really a C++ issue but seen in a C/C++ project.

The trickiest issue I had to deal with was an initialization issue when starting up the OS on our platform that lead to unusual crashes. It took years before we found out what happened. Before that we ran the system overnight and if it didn't crash, then it was normally okay. Luckily, the OS isn't sold anymore.