One of the most eye-opening papers in this regard was the integer overflow checking paper (which Regehr was coauthor on), which found that every library tested invoked undefined signed integer overflow. This includes SQLite, infamous for its comprehensive testing, and various libraries whose sole purpose was to check if an operation would overflow without invoking undefined behavior.

Belief that you are skilled enough to write C/C++ code that doesn't exercise undefined behavior either shows that you don't know what is undefined behavior or that you believe you are the best programmer to have ever lived.