In 1994, Green Day performed at Woodstock '94, a sequel concert almost as unnecessary as the movie Titanic II (which, by the way, actually exists). The band was booked on the South Stage, while the big headliners (including Spin Doctors, Allman Brothers, and Bob Dylan) were on the Main Stage. So the fans who came to see Green Day play were basically the young punks.
The field at the South Stage was a muddy mess, owing to rain and the general wear-and-tear of a festival that had been running for days. When Green Day came on, fans started throwing mud—and the band ate it up. They started a mud fight, flinging mud back into the crowd, swearing at them, cramming mud in their mouths, and generally having a good time—until fans swarmed the stage and a security guard smashed Mike Dirnt's teeth in. (He had to have emergency dental surgery after the show.)
Many fans look back at Woodstock '94 fondly, calling it "Mudstock '94" largely because of this incident. (Primus also got some mud during their song "My Name is Mud," though they weren't nearly as cool with it.) The concert was also a serious turning point in Green Day's career, as their album Dookie went from #19 on the charts before the show to #4 soon after. Here's one video ("When I Come Around") from the set, as the mud fight is just getting started. Stay tuned at least until Billie Joe Armstrong's guitar gets nailed by mud, but he continues playing (quite nicely) anyway.
And here's the full 35-minute set. It gets messy. Jump to 23:30 to see stuff getting well out of hand. By 29:30 Dirnt is lying in a pile of mud, but still playing. (He wasn't injured until later.)
Incidentally, Green Day turns 30 this year.