People falling over.
I've always found it utterly hilarious when people fall over, and here's a few instances that had me laughing with tears streaming down my face:
I was around ten at the time, it was in the middle of school holidays so I was allowed to stay up really late; so it was around 2 in the morning (like I said, really late) and my dad and mom needed some cigarettes, and the only place that was open that late was the garage/petrol station, so after I demanded to go with my dad in the car, we went and got the cigarettes.
Everything seemed fine until he was walking around the front of the car to get to the driver's side, which was when he just disappeared!
First I was in shock (as you would be) or at least I was for a second or two before I burst out laughing -I must have laughed for 2-3 minutes straight, and my dad still hadn't reappeared so my logical thought was 'omg, has he fell down a manhole and can't get back up' -which I may have sniggered a little at, but when he still didn't get up I got really worried.
Which is when he did get up.
Covered in oil, and red faced.
I laughed the whole time it took us to get back home, and made sure to tell ever that he'd fallen over when we did as soon as we walked in.
The other instance was when I was around 14-15, me and my mom were out shopping, she was walking in front and everything seemed normal.
That is until she just went down, like one minute I was looking at the back of her head and the next it was gone -I of course, laughed.
And didn't stop laughing even when she started shouting at me to shut up and help her up (yes she was still on the floor), people over the road from us were staring over at us, and she was so angry and red faced that it just made me laugh even harder, to the point I was slightly hunched over.
I did eventually help her up, and carried all of the shopping home while she nursed her hurt ankle and rather bruised ego.
I made sure to tell everyone when we got home, though.