20? Elites or non-elites? If you pulled 20-elites in a five man...well, as a DPS I would /applaud the healer for letting you die even though it mean I had a repair bill to pay myself. Cockiness and aggrogance lead to just as many wipes as stupidity and incompetance, imho.

But, how do wipes happen in most of the PUGs I've been in? Here's a pretty typical scenario: tank goes in and starts pulling everything, demands AoE, we AoE because we are "nubs" if we don't (according to him), he can't hold aggro, DPS gets creamed, healer gets creamed for trying to keep the DPS up because tank couldn't get aggro back on such a large pack of mobs...we wipe. Second try, same group: we go back in, tank pulls large again "I wanna get this instance done fast plz thx", we are reluctant to AoE as heavily because we know he can't hold aggro, so we DPS but more slowly, healer goes OOM trying to heal through all that damage on the tank, tank dies, group wipes. Rinse, repeat, but after three strikes, I'm out.

FYI, I'm not saying DPS can't cause wipes (they often do, particularly when two or more DPS in a group get it in their heads to get into a DPS race), but tanks who overpull can be majorly bad news.