Well, compiling C11 with a compiler that only supports C98, or GNU-C "extensions" in contrast to vanilla C, but there's no runtime compatibility issues like with interpreted languages (if that is what you were getting at).
Also, there's the problem with platform portability (in terms of compiler "oddities", not system libraries) which I won't get into, because it's a hairy subject. :)
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Well, compiling C11 with a compiler that only supports C98, or GNU-C "extensions" in contrast to vanilla C, but there's no runtime compatibility issues like with interpreted languages (if that is what you were getting at).
Also, there's the problem with platform portability (in terms of compiler "oddities", not system libraries) which I won't get into, because it's a hairy subject. :)