Maybe it depends on what you'd call a "bug". Even a flip-flop always has a nonzero probability of failure via metastability, but I wouldn't classify every system that uses a flip-flop as "buggy". Though to be honest now I'm not really sure what a consistent and useful definition of a "bug" is, if it's even clear and noncircular. (Maybe the best definition is "has a failure mode unaccepted by the users"? Not sure.)