Leading a group ride. I did it snowmobiling last weekend. One guy was all "Let's get 'em unloaded and go riding" as soon as we got to the cabin- not time for a pre-ride pointer discussion.

I should have just stopped and waited till everyone came back and listened, but whatever.
Needless to say about 15 miles down the trail along some power lines I went off the trail following tracks through powder and came to a big hill with a stiff drop-off. Of course since I couldn't see the other side, I slowed to look, then went safely (and slowly) over. Two guys did the same thing, then the next 5 proceeded to hit it full speed in the dark and smack their faces (helmets and all) against their windshields. One pretty good cut, two bruises (visors open), and two guys knocked to crap as they flew over.
This resulted in a 20 minute stop in our riding (instead of a 3 minute conversation at the start) while people assessed injuries and I explained a few riding rules- (namely don't go where you can't see, ride your own ride, and watch for the guy behind you). Of course you come off like a dick explaining some pointers AFTER injury occurred.

I love my friends, but sometimes a couple of them can piss me off a little. Someone has to go first, and once again I was reminded why it sucks being that guy in a big group (we had 10 riders). This is why I gave up group rides in the summer and stick to my three other core guys with whom we have ridden for 10 years or so.