I had the most epic thing happen during a session of D&D.
Situation: The big bad of the campaign was a corrupt politician, puppet master type. It was established earlier that we'd need to unearth his evil doings. We needed to tail him to some place we knew he'd meet up with some shady folks, and we learned he'd be passing through the (bustling) marketplace, ideal to follow him without getting noticed (by him, or his numerous guards).
So we're on the marketplace. Suddenly, our warrior (!) goes: "I discreetly walk up to the lord and attempt to behead him with one strike."
Our game master frowns. Then he smiles, and says "All right. Roll reflex 25 to get through the sea of people, then stealth 25 to do so without getting noticed by the guards, then land a critical to succeed the instakill."
We knew this would totally not happen - the only way he could pass the stealth AND critical checks were natural twenties (so that's 0.25% right off the bat).
20. The entire group laughs out loud.
20. Again, laughter. The GM goes "All right, you're there."
20. This time noone laughed. You could literally feel the epic in the air.
A split second of silence, the groups bursts into laughter. The GM just stares at the table in disbelief.
We pretty much skipped the entire second half of the campaign. Our GM salvaged it, though; Our buddy was now a criminal, after all he just killed some important guy in the middle of the goddamn marketplace. So from thereon, we were on the run. We barely survived the escape.