They annoy me too, the tasters of what is coming up. They are at the end of a program and either side of commercial breaks, and I prefer to have a surprise rather than have that ruined.
Sometimes, a clip is shown in the previous show, at the beginning of the current one, and several times before its real appearance. It's poor quality programming IMHO: take 20 minutes of filming and stretch it to 60.
Don't you just love it on the news, though, where, before each interview in the field, we have to have a demonstration of the interviewee's ability to walk?