My worst was throwing OB into a rushing river. Set the bag down on what appeared to be a safe spot on the side of the river to give a quick look. Step down about 2' from the bag only to be confused as to why all my discs are rolling past me and then my bag too!
Iphone in my pocket, jump into the river grabbing discs and trying to grab my bag. I completely submerge and come up to fall over again. Manage to save about 5 of my 20 discs and my bag. The rest are long gone. I'm obviously soaked to the bone and utterly upset.
Since we're playing tags (and on hole 3) I tough it out and play the worst golf of my life.
Worst I've seen in person is a toss up between these 2 incidents:
1. Guy on my card in the 2nd round of a very long course, throws a drive from a wet rubber teepad and has his plant foot give way. Simultaneously has his knee give out and his inner thigh go terrible. Dude could barely walk but finished the tournament.
2. Friend of mine, who shall remain nameless, throws a shot in the same tournament, different card. Tree about 15' off the teepad. Kicks pretty bad, but not worthy of the re-tee in his opinion. Jail shot -> tree. Worse jail shot -> tree into OB. I had to talk him off the DNF ledge because he was done. Tears in his eyes he tells me about how much work he'd put into the sport and I felt for him. We've all been there - and I think the feeling of having all your confidence drained is almost as bad as the horrendous score you'll have to have put down on the scorecard.