Great opening line. I really like the way that it's active and shocking, and immediately pulls the reader into the story.

I also particularly liked the narrative detail, explaining things like "it was the kind of joke that you have to pretend is funny because it's not." Those sort of narrative asides really worked well to convey the tone and detail of the piece.

The actual events of the story as they unfold are a bit confusing. Even rereading it a few times, I'm still not entirely sure what happened. It seems like a guy who looked like Max wasn't amused by the fake holdup and beat him senseless, but the cops were there so quickly, it almost seemed like he (inadvertently) tried to pull that prank on someone he didn't realize was a cop?

Also, the extent of his injuries (hospitalization, jaw wired shut, etc.) seem too excessive for all of his friends and family to be laughing about. It would be one thing if it was a stupid prank that caused an inconvenience, but this was a stupid prank that actually caused traumatic physical injury; it doesn't seem like the kind of thing people would be surrounding his hospital bed and laughing over.

Overall, I really enjoyed the story and think there's a tone of potential here. The style of writing and the general premise are a great combo and, with a little work on the execution, this could be a fantastic flash fiction story. You're off to a great start!