Two people screwing on the pavement in daylight in front of a crowd (they had most of their clothes on, though). This was an east coast late 80s show.
We were hanging out a van after a show waiting for the driver and a guy right outside the van was giving his girlfriend a hard time for flirting and so she took off her top and gave us a good view of her breasts to piss him off. East coast late 80s.
I went up to the top level of the Capitol Center in Landover, MD to find a friend during set break and there were like 10-15 totally naked heads up there. My friend said they had all been there dancing the entire set. The same show, there was a topless girl in the parking lot before the show. These struck me as strange considering this place also routinely had lines of cops with batons outside.
I saw totally naked girls dancing at California shows several times at the Frost and Cal Expo (85-90)
Billy Graham Presents had an official, in between set event featuring a ring of topless girls dancing with flowers (Frost 85 I think)
Then there are the fun people who get up on stage during shows. At Alpine 88, a girl got on stage and sat down in front of Jerry during Morning Dew (hey, seemed like a good location to watch from at the time). A guy also walked calmly across the stage at Furthur Bethel last summer and security didn't even notice until he was on the other side entering the crowd. There was also a west coast show in the 80s where a naked girl got on stage during the show and it was funny because they were unsure of how to grab her..
Frost 85 was a particularily entertaining show. There was some bad/strong/laced acid going around. A friend who has taken some had to be taken to the medical station after the show because he was seeing 'god'. There was a huge number of people at the station all having the same problem. The medics were totally overwhelmed...
Outside Pittsbugh (88 maybe), we got out of the show to find like a thousand cops in riot gear 'encouraging' people to get the hell quickly before they started beating us. I'm not even sure what prompted this but it was ugly.
Hartford Fall 79. I was inside the show about a half hour before the starting time and a roadie came out and told us the show was canceled because Jerry was sick.
Hampton 88 where some heads took crow bars and pried the doors off outside doors and poured in - people were already inside and they had take tickets so there was little they could do but the place was packed by showtime.