After 6 years or more of playing CoH there are too many examples of the "Holy ----" moments to go through for me to present them here. So I'll mention only one. Please forgive the length, for I have a tendency to be verbose.
Troll Mission, in an abandoned office building, don't remember the name of it. I'm playing an invulnerable/broadsword scrapper Celtic warrior with bare, tattooed upper torso and kilt named Asteryx. The team had just cleared the first floor, taken the elevator up to the second, and come down a hallway to an opening; u-shaped bend to the right, open doorway to the left that led to a staircase that you could take to the landing above. A walkway led from the landing across the u-shaped bend on the right. The team had just engaged the first group of trolls near the bend when some dimwit ran up the stairs on our left, bumped full into at least two groups of trolls on the landing, jumped down from it into another group on the back side of the u-bend and led them ALL back to the group. Everybody died.
Except Asteryx. I had popped a purple chiclet and a green when I saw that mob round the bend and was flailing away, surrounded by a sea of enemies. I was cursing the nitwit who was getting us all killed when I noticed a bunch of chiclets starting to appear in my tray. I happened to be standing on top of one of the team's female characters, and she was feeding me purples and greens and whatever else she had. So I fought and spammed multiple-target attacks for about 5 minutes. By the time the team members returned from the hospital almost all of the trolls were dead. The most memorable experience I ever had in the game.