In my time gaming, I've had two groups implode:
The First Story: I was running a d20 Modern Dark*Matter campaign. This campaign had been happily running for about 9 months, gaming weekly. There were three PC's, and I was quite happy at the high roleplaying/low powergaming ratio going on. A friend of one of the PC's, who was just starting to come around our gaming club wanted in to the game, and I was certainly wanting more players.
She was a tall, blonde and blue eyed knockout. She was hot and she knew it. She had a boyfriend, but made a point of hitting on me at every session. Her boyfriend even came along to the game once, but he seemed utterly disinterested in playing (or her), and the feeling seemed mutual from her. I spurned her advances: I was
not going to fool around with somebody elses girlfriend, just not happening.
She was new to gaming, and was having trouble getting the whole "in character" vs. "out of character" thing. I don't think she was entirely all there mentally to begin with, since I understand she only came to college (and found our gaming group) because her attempt to enlist in the Air Force was declined on psychiatric grounds. In a conspiracy themed, high RP game, characters may well have secrets and lie to each other, and not everybody is one big happy family who goes together to kill orcs and take their pies. Thus, when a character kept a secret from her character, or lied to her in-game and she knew it was a lie out-of-game, or did anything unfriendly to her PC, she took it
very personally. This started to build tension at the table, but I figured since she was a new gamer, I'd let it slide and try and educate her about roleplaying.
Then. . .
it happened. She broke up with her boyfriend, and since she didn't have a car of her own, she got her roommate (one of the PC's in my game) to drive me over to my place in the middle of the night. She wanted to "talk", or more properly, now that she dumped her boyfriend I didn't have an excuse to say no and she was there to seduce me. I tell her I don't want some empty stand, I'm looking for a serious relationship and I'm not going to do much on the first night, one way or the other. The accepts this, and we cuddle that night (and not much else), and she goes back to her apartment in the morning, promising that we were a couple now and wanting me to call her tomorrow.
The next day, she doesn't pick up the phone, and over the next week she doesn't answer e-mails, reply to IM's, or anything. Her roommate is gruff and terse and says she's been gone a lot, and that he's very unhappy with her. The game the next week is cancelled because of an unrelated tragedy: a member of our gaming club was killed in a car wreck and the funeral was scheduled for when the game normally would be. At the visitation, she's clinging to the arm of another member of the club, and acting like nothing ever happened. As I piece it together, right after she left me, she went to every other single male player in the game (all three of them, including her roommate) and tried to seduce them, then went around almost daily to other guys in the club before alighting on the shoulder of another one, and they got engaged after being together about a month, and they generally stop hanging around the gaming club to be with each other.
After that, the game kinda disintegrated, as the whole affair with her left a really bad taste in everybody's mouth, and that game reminded us all of her.
The Second Story: This one is much more recent, it is the game mentioned in my sig

. It was a Forgotten Realms 3.5 game that had been going on for about 8 months. I had a core of 4 dedicated PC's who were there regularly, and another 3 PC's who were there occasionally. One of the players was a girl I met in my martial arts classes (Girl #1). She was attracted to me and the feeling was mutual, and we dated for a short time before she decided to "just be friends". I was cool with this, and we quite maturely set aside romantic intentions for friendship and kept gaming. Now, about 6 months later another girl in my group takes an eye to her. This second girl is an old friend of mine I've known for about 8 years, and was one of the people I first gamed with and first introduced me to D&D (Girl #2). Both of these girls are openly bi, and they apparently start dating at some point after the game has been going for about half a year. Apparently Girl #2, my old "friend" was somehow jealous of my prior relationship with her new girlfriend. We were having a normal gaming session, which was wrapping up like usual, when I start to call it a night, she says that she has a few other things to do.
She's writing furiously on her notepad, and steps into the bathroom for a minute. She comes back and hands another PC (the party leader) a note. The two girls get up and are about to leave, when my "old friend" idly suggests showing me the note (literally standing at the door and opening it) so I know what's going on in my game.
It's an in-character note from her character to the party leader, saying that her PC (and the PC of her girlfriend) are eloping, they're taking a share of the party treasure (which she had already marked off the sheets since she was party treasurer). The note is vague, so I have to ask if this means they want to retire their characters (she had talked often of retiring her character and creating a new one, because she has so many concepts she wants to play) or if they are quitting the game. She just answers "you're a good GM, but I've just got other things I want to do with my Saturday nights". Girl #1 just gives a sheepish, and somewhat apologetic smile as she walks out the door. Admittedly she did often want to end games early so she could go out clubbing with her new girlfriend, but we'd even moved the game to Saturdays on her request because the prior game night conflicted with her therapist appointments.
I'm pretty devastated. It gets worse when my "old friend" pretty much severs all ties with me, e-mails me asking me to drop off several things she's left over at my place over the years (or loaned me indefinitely) at a mutual friends, and strikes me from her friends list on her friends-only livejournal. Wondering what the heck is going on, I go to Girl #1 and get a straight answer. This second girl is always quite level headed and fair-minded (reasons I liked her

). She explains that yes, my "old friend" was quite uneasy with spending time with her and me together (apparently the idea of any residual feelings between us makes her very insecure), and that I'm still welcome to hang out around her (Girl #1), but Girl #2 just can't bring herself to spend time with all three of us in once place (and for somebody who is 25, is acting an awful lot like a teenager having high school petty drama). The only reason Girl #1 agreed to this (she objected to it in an apparently public argument in front of her family, even her mother commented on it to me when she saw me) was that I had enough PC's to keep the game running, since Girl #1 didn't want to ruin my game (she enjoyed it greatly, it was her first game other than 1e AD&D). She was quite upset when she found this wrecked the game, and made a point of letting me know she was still my friend and she was trying to do what she could to keep both of us happy. I at least appreciate the conflicting position she was in, and don't blame her since she didn't intend to destroy the group.
This unfortunately, pretty much devastates the game. They were 2 of the 4 PC's who regularly were in my game, a 3rd was a friend of Girl #2 who was in the game at least partially to spend time with her. Then one of my occasional attendees tells me that he just got hired at a law firm (having passed the bar a few months before), so this leaves me with one regular PC, and 2 occasional PC's who show up roughly once a month. Thus the game imploded, making it the second time in two years I've had a gaming group go down in flames due to relationship related drama.