20 Things You Didn't Know About It's Always Sunny In Philadelphia
6. There's An Unaired Season Six Episode, "The Gang Gets Successful"
An episode was originally written and filmed for season six called "The Gang Gets Successful", yet which was never actually aired on FX and to date hasn't ever been released to the public.
However, much of the episode's material was ultimately repurposed into season seven's "How Mac Got Fat," allowing the episode to show "new" material of a thinner Mac before his weight gain.
Footage from the "lost" episode was even aired on a promo for the sixth season, though for reasons unknown it never made it to air in its original form. Clearly, though, they found a better use for the bulk of the material.