^ I've seen/heard it happen quite a few times. As the server's partner, I will sometimes hear the ball nick the net tape, but the receiving team does not acknowledge it. It has happened a number of times that both the server and partner has heard the net contact, the returner's partner did not say anything and, subsequently, the returner (who possibly did not hear the contact), hits the ball back for a winner -- because the serving team did not react since they relaxed after hearing the "net".
The best way for the serving team to protect themselves in this scenario is to call the "let" on themselves. Cindy's take on this is spot on. OTOH, the returning player or team can "steal an ace" by claiming that the ball clipped the net. For this reason, Div 1 men's tennis no longer has a '"let" for balls the hit the net and land fair.
Every tool is a weapon -- if you hold it right. (~Ani DiFranco)