It's "All the world's a VAX" in its new form, where you depend on some language/library feature that's actually not in the standard (IIRC alloca and preprocessor extensions are common culprits).

And then suddenly you're on a different platform and discover that you can't rely on that -- pretty bad if it's in a central part of your system (like trampolining functions for your toy lisp).

On the other hand, you might run into the same issues if the standard support is sub par. People used to declare K&R-style functions for ages after ANSI was passed, and I wouldn't bet a central part of my system on C99.