hmmm tricky, there are some tracks that give me problems, but I'd have to say some of metallica's riffs, disposable heroes is a good example, are just so fast. I think anything that's highly polyrhythmic gives me the most hassle because on my own, it doesnt sound too bad, but playing along to future breed machine by meshuggah it can sound so sloppy if you mess up, in fact almost anything from the 'destroy erase improve' album is a nightmare, they use time signatures like 5/19 and other weirdness, and they switch times like nobody's business, if you make a mistake playing along with those guys it stands out really badly. So I would say as opposed to lightning fast chops, I would say tight enough to turn coal to diamonds rhythm playing is my greatest fear (And I pride myself on my speedy wrist, hell I'm a death metaller, it comes with the territory)