Breakage of existing code is common.
There was quite some discussion about the fact that a new version of GCC optimized checks away that depended on unspecified behaviour of integer overflows.
That was around april. The CERT advice is still there but has been revised multiple times (easy to find). The fun thing was that GCC got blamed for beeing an unsecure compiler while tons of other compilers applied the same optimization as well.
If you want a fun read/flamewar google for "Cert GCC". You'll find lots of discussion if this was a bug in the "optimized away"-code or a bug in the GCC-optimizer. Even the C-experts weren't sure who to blame for a day or two..