My craziest experience was a soccer game in Mexico... the León - Tijuana game in the Mexican Promotion Playoffs (Liga de Ascenso) in 2011. Basically, León was one of the biggest teams and fan bases in México but had spent ten years in the second division so their fans were absolutely desperate to get promoted back to the first division. León had dominated the entire season and were huge favorites to move up to the first division, but if you lose in the playoffs you are still eliminated even if you were first in the regular season. León had already lost the road leg of the 2 game series 1-0 so they needed one goal to tie the series back up (and advance since the higher seed advances in the event of a tie). Anyway, León basically started out dominating the game, but allowed a goal against the run of play to fall behind 1-0 after maybe 15 minutes. Then their star forward got red carded, leaving them down a man, for an idiotic and extremely violent elbow to an opponent's face right in front of the ref. In spite of being down a man León kept dominating but hit the post a couple of times and went into the locker room down 1-0 (and down 2-0 on aggregate). To start the second half, León hit the post again didn't score... and that's when things started to get crazy.
The ref got a cramp and stopped the game for like 10 minutes so that he could lie down on the field and stretch. While this was happening, the already desperate and infuriated crowd started going nuts and trying to climb over the barbed wire fences surrounding the field and started throwing bottles and cups of beer and piss onto the field and at the police guarding the field and lots of people started fighting with the police. After maybe 10 minutes on the ground the ref finally got up and signaled for play to start again... it was a goal kick for León and he blew the whistle for them to start play. León's goalie, who was in a big hurry after the ridiculous delay, comes out and inexplicably restarts play by passing the ball straight to the other team's center forward. The Tijuana forward pretty much walks in and scores to make it 3-0 on aggregate and shit just absolutely hit the fan. A bunch of León's players attacked guys on the other team or pushed the ref and maybe 2 or 3 of them got ejected basically ending any chance that León had at getting back in the game. Meanwhile, in the stands, the violence got completely out of control as the fighting between fans and police escalated quickly. The section I was in wasn't quite as crazy as some of the other sections but there were still people trying to climb onto the field and a guy a couple people down from me was having his wife and maybe 10 year old kid look for glass bottles so he could throw them on the field or at the cops guarding the field. Tons of people where I was were throwing glass bottles at the police and trying to break the fence between our section and the field. In other parts of the stadium, plenty of fans started making it onto the pitch meaning that the game had to be stopped for maybe 15 more minutes and in other parts of the stadium where the violence was getting even crazier the police were letting people go through a door in the barbed wire fence surrounding the field so that they could escape the stands, cross the pitch with play stopped, and leave the stadium through the tunnel rather than having to use a normal exit where people were brawling really hard. I also saw tons of people literally take off their León jerseys and light them on fire. The number of people whipping glass bottles at the police or into the crowd, physically fighting withe each other or with the police (who didn't have guns - just clubs) was absolutely crazy. My girlfriend, who is actually from León, was absolutely terrified and the scenes were pretty heartbreaking for her so once it seemed safe enough to head to an exit, after witnessing maybe 20 minutes of this mess, we left. As things were not much good was going to come of watching her team play 7 or 8 on 11 already losing by three goals. Anyway, it was absolutely nuts.
Aside from that story, the other two crazy ones I have are somewhat similar. I was at a city baseball championship in Guanajuato, Mexico, where I live, where the crowd got really out of control and people started whipping glass bottles at each other and brawling in the stands. That was crazy since I knew a lot of the people on the field and in the stands... luckily I was watching the game from the dugout so I was out of harm's way but a number of people were somewhat seriously hurt and a few people got arrested. Once the violence started the players and I all went out on the field to watch and to help bystanders climb onto the field to escape the violence. I saw a guy in the stands right in front of my try to attack a policeman with a beer bottle and get clubbed in the head. My friend's cousin got nailed in the head by a guy's belt buckle and needed 12 stitches on his head.
The other craziest experience was being at the USA - Mexico soccer game in Chicago for the Gold Cup final in 2007. There were maybe 5,000 Americans and 60,000 Mexicans at Soldier Field and we were in an all Mexican section (aside from my group of 7 friends - we were all supporting the US). The US won 2-1 and as the game ended tons of Mexicans threw beer and coins at us... and a Mexican fan whipped a beer bottle at our car and then kicked a giant dent in our door later as we were leaving the stadium. Truthfully, most of the Mexican fans there were really nice to us and even as stuff was being thrown at us the Mexicans near us were apologizing and saying that we aren't all like that, but it was a crazy experience.