People hurting themselves will always be funny.  And sliding is a really easy way to hurt yourself.  Check out the highlights of people wiping out on Slip N Slides, regular slides, water slides, and some other slides too.

 

I'm thinking the slide at :56 is the funniest, but that's just because the dude is really fat.  The escalator slide at 1:55 is funny as hell too, just cuz you know that kid thought he was the man, and then he's on his head.  The last one is barely a fail, considering how long he grinds that rail.