The "slap fight" occurs during the tailgate between Charlie and Mac. It is a masterclass in the show's physical comedy.

While Dennis, Mac, and Dee try out, Frank and Charlie get distracted by a tailgate gone wrong. The Iconic Slap Fight

The following is a breakdown of the It’s Always Sunny in Philadelphia episode "The Gang Goes Invincible" (S3E2), often remembered for the iconic "slap fight" between Charlie and Mac. Episode Overview Season 3, Episode 2

Frank accidentally drops acid (LSD) and gets trapped in a portable toilet, leading to one of the show's most famous visual gags.

Dennis, Mac, and Dee fail miserably; Dee is actually the most talented but is dismissed because she is a woman.