Perpetually single Penny may not have seemed the most likeliest candidate on “Happy Endings” for engagement, but that’s exactly what she got. And in true “Happy Endings” fashion, it was an event filled with shenanigans, or in this case, pranks.
After getting Max on a lottery prank, he vowed revenge on the rest of the gang. The rest of the episode saw Max pulling off hilarious and elaborate pranks on the entire gang, but one of those got misinterpreted. Penny walked in to find rose petals, a beautiful display and an engagement ring in her apartment. Then she got slimed.
Understandably, she thought the engagement was part of the prank, but it wasn’t. Pete had set that part of it up. By the end of the episode, Max had blown up his own limo, which had been filled with popcorn in an attempt to prank him. Thus, the stage was set for a memorable proposal.