One night while coding half asleep, I wrote the following to increment a variable in C++
x = x++;
The problem with this code is that it is an undefined behavior. It looks okay at first glance, and then when you consider the machine code that would be built from it, a bit of ambiguity arises. The problem comes in with the = sign vs the ++ operator. Both of which are assignment operators for the x variable, but it is not well defined which assignment should happen first/last. The code in use was actively being used in both MSVC and GCC environments, each producing opposite assignment ordering. This was awesome to debug, since the code "worked" on one platform but not the other!