There's a scene in the movie "Rounders" where a bunch of professional poker players from New York City are in Atlantic City and end up at the same table with each other. Then some non-pros sit down and, while the pros aren't colluding with each other, they know they have a better chance playing hands against the amateurs than against each other, so that's what they do...
A guy from Las Vegas was in St. Louis recently and sat down in our $5-10 no limit hold'em game. Naturally, the conversation turned to where he usually played in Vegas. He talked about the action at Aria, the Encore, and the Venetian. Someone asked him about the Bellagio poker room and he replied that he still played $5-10 there occasionally, but won't play in the $10-20 game anymore because, too often, there are players at the table working off the same bankroll. That means that they've agreed to share their winnings and losses, and because they're not playing against each other, they're more likely to team up on you whenever you get involved in a hand with them. If there are a couple of these teams at the same table, that makes four out of nine players who have a distinct advantage over anyone playing on their own (which we all should be).
I've been at the table when two opponents were obviously playing together, making moves to push other people out of pots, even when they didn't have much of a hand. The worst example I remember was in 2010 at the Venetian, a stop on the PokerStars North American Poker Tour. I was in a pot-limit Omaha cash game at about 3am, playing four-handed, and I knew one of the other players, John, who was from St. Louis.
That's when Young Guy One sat down at the table. He played fairly tight poker for about 15 minutes until Young Guy Two sat down. They barely looked at each other, but both of them started getting way too aggressive for this game, pushing people off hands and taking down several pots in a row. They were especially sticking it to John, whose losses had put him on tilt. He started playing more hands than he should have and they kept playing right back at him. However, when they had gotten him out of the way (for instance, raising and re-raising so much on the flop or turn that he had to fold), they stopped betting against each other. They even laughed as they showed their hands, and we could see that neither of them had anything decent most of the time
After a half hour of this, I'd had enough and called over the floor supervisor, told her I thought these Young Guys were colluding at the table, and explained what I'd seen. She turned to them and asked if they knew each other. There was a pause before they both quickly picked up their chips without saying a word and left together. John was pissed off at me because he wasn't going to get a chance to get his money back, but I explained that he was at such a disadvantage against those two that it wasn't going to happen.
On another occasion, I was accused of collusion. It was during a tournament series in Tunica, Mississippi, and there were a lot of other people from St. Louis there. After busting out of a tournament, I put my name on the list for a pot-limit Omaha cash game. I was eventually seated at a table with Alan and Debbie, two very good players from back home.
We were talking about our experiences in some of the tournaments and other cash games when someone at the table that we didn't know -- who had been losing because of his terrible play -- said, "I knew it. You three know each other. It's obvious that you're playing together against the rest of us." I responded, "If you think we're playing together, you're not paying attention. Debbie would like nothing more than to take all my chips, and I'd be ecstatic if I got all of Alan's chips. That's true in St. Louis as much as it is here in Tunica."
Alan and Debbie laughed at this as another player we didn't know chimed in, adding, "They're not colluding. I've watched every hand at this table, and I haven't seen any evidence they're playing together. You're just mad because they're better players than you are and you keep giving your chips away." That really put the loser on tilt. On the next hand, he was all in on the flop with a non-nuts straight draw, and when it hit but he lost all his chips to someone with the nuts (who was not Alan, Debbie, or me), he stormed away from the table.
By the way, Debbie took about $1,200 off of Alan and me that day. And she never offered to split it with us.