The oxymoron "safety by accident" came up in a conversation. It sounded very natural in context when we first mentioned it, and then we stopped for a second and realized it was kind of silly. The idea was that some code was flawed, but some unrelated thing just so happened to conveniently prevented the bug.
Code like this can be a maintenance nightmare; because while the behavior may be technically correct for current usage patterns, it's very brittle as usage patterns change.
Here's a simple example of 2 bugs nullifying each other:
static bool IsPositive(int x)
return x < 0; // bug #1, wrong check
static bool IsPositive(string s)
int x = -int.Parse(s); // Bug #2, parse gives inverted value.