Years ago, my wife had a bug that only happened in a non-debug build and only on one specific system. It was a recursive tree traversal algorithm, written in C, with an OB1 error in some pointer arithmetic. If memory was allocated in a particular order, the function would read off the end of level X's data and find level X+1's data, which was structured exactly as expected, so it would end up processing certain nodes twice. In the debug build there was a debug variable being initialized to null, which was in between the two blocks of data and therefore got interpreted as a terminator.