I don't remember many experiences that have made me laugh when playing alone. When I play with friends though...
In GTA4, I was in free play with a friend just waiting for some other friends to show up, so we decided to race and see who could get to the bottom of the map faster from the top. I grabbed a helicopter, and he took some really fast car. Near the end of the race, he somehow manages to go flying into the water and ends up getting stuck on a garbage boat. He asks for a pick up, so I go down with my chopper to try and land on the boat, but end up hitting him with the propeller and sending him flying back to land instead. At least he made it back to land.
When playing Crackdown, me and a friend were orb hunting when I found one in a small room inside an underground tunnel. I told him I found one, and to come to where I was. Before he got there though, I filled the room with remote bombs. When he walked in, I asked him "Did you get it?" He replies "Yeah, but what the hell is that sound?" Then I blew him up just to hear him yell "What the fuck?!?"
Back when I was trying out Animal Crossing for the DS, a friend invited me to his town so we could trade shit. He went inside his house for a minute to do something, so while he was in there, I took my shovel and dug holes around the door. When he came out, he was like "What the hell? Where'd all these holes come from?" Since you can't jump over holes, he had to go back inside to get a shovel and fill them up before he could leave his house.
And lastly, I was playing Saints Row 2 with a friend of mine and he went to go get something to eat and said he'd be back in about half an hour. I think it's pretty obvious by now that I like to screw around with my friends (hey, they do it to me too), so he left his character on a little ledge where I couldn't run him over or anything and took off. To be honest, I couldn't even find his character for a while, since the ledge was semi hidden and his character was in some bushes. Once I found him though, I grabbed a pickup truck and backed it up to the ledge and kicked him into the back. Then I drove him to the water and grabbed a boat and kicked him onto that (which took like ten minutes cause he kept falling off the boat). Then I drove him out to the furthest little island I knew about and kicked him off the boat, leaving him stranded. After a while, he came back and was like "Well, I'm back and... what the.... Oh look... I'm on a fucking island in the middle of nowhere, what a suprise. How the hell?!" The only downside is that for all my work, I forgot that he could just teleport back to the mainland.