There have actually been few such moments of either kind in the debates so far. I certainly think Brownback, Huckabee, and Tancredo raising their hands to signify not believing in evolution was a bad move, but they were not serious candidates anyway. Giuliani almost had a great moment when he challenged Ron Paul on blaming American foreign policy for September 11, but Ron Paul stood his ground and is certainly 100% correct that the war was undeclared and is therefore unconstitutional.
On the Democratic side, Edwards' taking Hillary and Obama to task for not leading opposition to the recent war funding bill was also a good, but not quite a great, moment - mainly because Obama had an excellent rejoinder. Obama has been against the war from the beginning - unlike Edwards - so Obama was able respond to Edwards that at least it didn't take Obama 4+ years to come around.
Which raises an interesting question - what could Edwards have done to make his upper cut more effective?
Better preparation is almost always the answer. The Edwards team should have realized that Obama could respond as he did. And, taking that into account, Edwards could have confined his critique to Hillary - who, as the front runner, would have been the better target in any case. Obama would likely have stayed out of such an exchange, and given Edwards a clearer success.
Many things of course become clear in the wisdom of hindsight. But given the stakes in these ongoing debates, candidates need to be even better prepared than they are now...
Going to the Candidates' Debate
Third Republican Debate