Well yes, there's the huge "GNU C(++)" dialect of course; if nothing else I think Clang's existence has helped there although of course it tended to adopt a lot of the things outright in the past. This is really something you always get when there are multiple non-reference implementations of something and one has significantly larger market share.

But, I think a more relevant example might be people who write code prefixed with comments like "Destructure this manually. At -O2 on GCC this allows the first field to be placed in a register" which is putting waaay too much dependence on the internals of a specific implementation, beyond even whatever extensions a particular compiler claims to support.