Oh, so many.
Battlefield 1942: In an online game on Omaha Beach, this one guy kept driving down the same road. I was in a trench maybe 50 metres away from said road, but I knew the timing on the server, and figured out the exact moment his tank would be coming down that road. I threw a grenade blindly into the air, and it landed in the hatch of his tank.
I used to never play Engineer, but one game I did. A German car with 2 people in it was trying to run me over. I threw a mine at it and it blew up the car nicely. I turned around to see the flaming hulk land, and noticed a guy pointing an STG 44 at me. I pulled out a rifle, and as he started firing, the remains of the car landed on him for the kill.
In a game on Iwo Jima, I saw a great opportunity in the form of 10-ish Japanese soldiers running through the same chokepoint. Someone else pulled off a million-to-one shot by hitting my plane - just before the strafing run hit - with a Panzershrek. My plane was wrecked, but the wreckage sort of glided through the air. It pulled off a million-to-one of its own, bowling over 8 of the soldiers I had targetted.
At the battle of Britain, I snuck into the German hangar to steel a Ju-88 bomber. I went into an inescapable tailspin while dodging fighters, so I bailed out into the ocean... and looked up to watch a German tail gunner jump out of his own bomber, climb into mine, and fly to safety.
Battlefield Bad Company: My teammate, a sniper, was saying "Oh, I'm dead, 1 health left and a heavy tank coming right for me". I couldn't even see the tank, and I was in another base maybe 1 kilometre away (this was on Harvest Day), but I fired a no-scope rocket shot toward the spot where I last saw that teammate. The rocket reportedly hit the tank just as it was going to run the guy over.
Team Fortress 2: I was scoped in to snipe, when a hostile Scout started shooting at me. I spun to fire at him and missed, killing the invisible Spy sneaking at me. It was my only headshot ever. I'm terrible at sniping in Team Fortress 2.
Toribash: There's a game mode where you fight on top of a tower, and whoever touches the ground first loses. I've had many wins where we both get knocked off the tower, with one or both of us in several pieces. On multiple occasions, I've only stayed off the ground longer by catching my body parts and holding them up.
In another tower mod, I found myself wrestling my opponent in midair. We hit the ground at the exact same time, to get a tie. Twice in a row.
War Rock: I meant to switch weapons (Everyone runs faster with the knife!), but accidentally fired my rifle in the air, towards the enemy base. 10 seconds later, I get the first kill of the round, a headshot on some hapless enemy pilot. The bullet must have arced through the air and travelled for 5 kilometres to reach the enemy base.
Another time, also on Conturas, I set my plane upwards and hoped to get to the bathroom and back before I crashed. I came back to see myself dead, congratulations from my teammates, and insults from the other team for being a "rammer noob". I asked what happened. It seems that my plane had crashed into a fully-loaded enemy helicopter that was in the process of base-raping us.