audio dubs onto video are certainly common...for instance, in the movie "Dr No," Ursula Andress had such a thick accent that they dubbed every single line of hers. If you look closely (or even not that closely) it should be readily apparent. Of course, if such a detail is enough to ruin your enjoyment of an otherwise perfect bit of cinema, then there are bigger issues.

with modern video editing techniques and digital video and audio manipulation capabilities, such sloppiness is really relatively unheard of in something even semi-professionally edited. Give me an hour with adobe premiere and I can take a verse out of a song and make the video line up so that nobody is the wiser, even on a careful listen, and I'm not even a pro.
in general, the process is to make the audio track(s) and then to splice the video onto it. When switching from camera to camera one simply has to make sure the timecodes line up, and magically it looks seamless. Of course, if the tempo is slightly different between different takes, this creates complexities, but again, modern editing technology makes this easy--find a landmark, such as the beginning and end of a phrase, and then scale the video clip to match that. hard to do on film, takes less than a minute on digital.