It has mostly happened to me when the show time changes beyond the limit. The schedulers seem to try to avoid the word RAT and usually just say "just giving you a heads up your showtime has changed" Back when RAT's only paid 110% my usual response was, "if this is a RAT I'll decline the trip and remain in sub" RAT's are now 125% so it might be a bit more appealing to accept the trip. Just make sure they admit it is a RAT and confirm your choice of taking it or going back to sub. I've found that a very friendly tone from scheduling usually means there is an overage opportunity.