I've, unfortunately, had many experiences in public toilets. Most of the post here seems like standard things in my opinion. Fighting, Sex, Drugs and turds in the sink? I think I've been guilty of all of those a few times.
My worst (and I warn you now, it's not pretty or nice to read) would be finding someone with both of her wrists cut, blood pooled all around them. I still have the horrible image of that nice, very clean, sterile white tiled bathroom covered in blood stuck in my mind to this day and still have nightmares about it too.
My most humorous would be at my old job. Now it's not technically a public bathroom anecdote, I still think it fits here though.
It was fairly late at night and only a few of us were still working when the doorbell/security entry thing started buzzing. My boss and I were both equally puzzled to who it could be at that time of night so we checked the monitor/intercom. It was a lady, about 30ish, quite 'well off' by appearance, looking very normal and overly embarrassed. She explained that she was very sorry and realised that this was very out of the norm, but wondered if she could use our bathroom. After a little chuckle we decided to let her in to relieve her embarrassment, and herself presumably. She used the bathroom and came back out five minutes later, thanking us both very much and was quite apologetic before leisurely strolling out
Shortly after she left, my boss went in to the can only to reappear a few seconds later screaming "oh my god". She had shit all over the bathroom, the floors, the sinks, the bin. She had then wrote 'Bastards' on the wall in her feces.
Another time, while I was walking from Lands End to John O'Groats (southern tip of England to the Northern tip of Scotland) I arrived, quite late, in East Kilbride (just outside of Glasgow) I didn't have anywhere to sleep and it was too late to try and find a park where I could pitch a tent. I decided to use a public bathroom, lock the door and stay there until it was light. Only, when I entered the bathroom, lo and behold, there were 3 homeless guys playing a game of monopoly! In the end I joined them and hung out with them for the whole night. Very fun night too
I'll throw this one in for gross factor too. One night at a club myself and one of my good, but crazy friends, went for a piss. In the urine flooded urinal someone had been sick; That sort of sick where you've been drinking lager and blackcurrant and it's all thick and pink. Anyway, I made a dare with him that if he ate some, I would pay for everything all night. At first he was dubious, but another randomer inside the toilet also got on board and added £50 to the bet. Needless to say my friend ate a good handful of it resulting in himself then throwing up, then me and the other guy throwing up.