Wet Balls
I saw a discussion on the OOAK forum about whether it's a let if the ball is wet during a rally, and so slides off your racket into the net. The question comes up all the time. The answer is yes. Here's the rule:
Rule 2.09.02.04: [Play may be interrupted] "because the conditions of play are disturbed in a way which could affect the outcome of the rally."
If a ball is wet, "the conditions of play are disturbed in a way which could affect the outcome of the rally." The only question is how to judge this. A wet ball normally slides off the racket into the net. But so does a misread backspin serve. So it's a judgment call. The best indication is if there's a wet spot on the racket - but again, it's a judgment call since that wet spot might have been there before the point started or have been hit with sweat during the point. But normally it's pretty obvious if the ball went into the net because of a wet ball, and checking a wet spot on the racket is just verification. I have had opponents put my heavy backspin serve into the net and call "wet ball" when they had simply misread the serve, as indicated by their racket angle. (If you serve backspin but the split second after contact pull your racket up, this'll happen a lot. But it takes practice.)
I believe the wording used to be that it would be a let "if the ball was fractured or imperfect in play," but at some point it was changed. With both wordings the main reasons for calling a let because of the ball is because it is wet or fractured. (Or, of course, if another ball rolls into your court.)