Honestly most of my commission experience were lovely on the client side and a nightmare for me internally (depression is a helluva drug).
I did have ONE near-miss though, whoooo boy... So I was active in the competitive scene for this one video game. I didn't FLAUNT that I was a girl but you know, when you play with me, it's kinda obvious. I have a female voice. (I even have a female name - Mallory - but for some reason most guys who did not know me assumed it was male. I got a lot of "How are you, sir?" when I was a tournament admin, lol. Anyways). So for the most part, it is common knowledge that I'm a woman (or a "girl gamer" to anyone who made a big deal about it) and I don't try to hide it.
One day this guy adds me - and I've seen him around, I don't know him but I know he's also a competitive player. He says he saw my art and loved it and would like to commission me of a drawing of his character. Ok, cool. I've done a few of these. I like doing them. So I gladly tell him yes.
Then it gets a little weird. He starts talking about how loaded he is, how much money he has, how he's gonna pay me $300 for this commission (my asking price was $50 and even that gave me heart palpitations). So I'm a little O.o about it but whatever. Maybe he won't actually pay that much, or maybe this guy really knows how much artists get underpaid and has extra money lying around so he wants to correct the injustice. I've had a couple friends over the years who came from old money for whom spending a few hundred dollars on someone else was like, couch change, so I kinda shrug and start working on his request.
It gets weirder. I sent him some sketches to peruse and sign off on, and he starts messaging me with "I like it, but can you push yourself and make it darker? I know there's darkness within you!" Keep in mind that at this point in time, my art looks like this:

... Yeah. Ok.
Unfortunately that's where the "weird client" territory ended and bat**** began. He begins to message me daily about this "darkness". Then he begins to message me daily about random topics of conversation. Then he begins to insist that I should invite him to hang out with my friends/teammates, and that I should invite him to "sit in" on my team's practices because "he's also a competitive player so he can teach us stuff" (at this point in time, I play at a higher level than he ever did. There's nothing he can teach me). Then he proceeds to stalk me from game to game and I even see him attempting to join a password-protected game I'm in.
Then he finds out I have a fiance and goes off the rails and starts sending me massive daily screeds about how all women are hoes and gold-diggers and "he would know" because he's a "big-shot lawyer." I'm like ok buddy, I'm p sure you're a paralegal pushing photocopies at some small firm. Then I block him, and show all of my friends/teammates the insanity. A couple of them apparently know him and are amazed that "he would do something like that" and I'm just like, gents, this is him. This is your friend doing "something like that." Keep that in mind for the future.