The Best Time Mac Broke Character On It's Always Sunny In Philadelphia

In the Season 9 episode "The Gang Gets Quarantined," a severe flu outbreak sweeps through Philadelphia and forces Mac, Dennis (Glenn Howerton), Charlie (Charlie Day), Dee (Kaitlin Olson), and Frank (Danny DeVito) to hole up in Paddy's Pub to wait out the epidemic. Yet, despite their best efforts, each member of the gang falls ill and is confined to the Paddy's Pub bathroom, where their symptoms get worse and worse as the episode goes on.

When Mac first enters the bathroom, he finds Dennis and Dee already in the throes of horrific illness, with Dennis vomiting in the sink, and Dee as pale as a ghost. Apparently, watching his co-stars act so absurdly sick made McElhenney start to laugh in plain sight of the camera, as noted in this Reddit thread.

Hilariously, McElhenney's entire body is reflected in the mirror in this single reverse shot of Dee gagging. There's something so funny about the little grin on his face, and the way he's just kind of standing there, clearly not in character at all. We don't know if this was an oversight on the showrunner's part, or if they found it so funny that they simply left it in, but because he clearly thinks he's off-camera (and is just giggling in plain sight), it's one of the funniest bloopers that has ever made it on the show.