DVR users, are you constantly developing your ninja skills?

I have a DVR system and I love the way I can fast forward past the adverts. Sometimes I’m ‘in the zone’ and can judge the end of the adverts really well. Sometimes I just can’t seem to get it right. Are the TV stations fighting back, putting hidden cues that make you THINK the adverts are done, and then putting something like a preview of another program and then a couple of more ads in? Or, am I just having a bad day with my DVR ninja skills?