There was the infamous party hat duplication glitch back in RSC If you haven't seen/heard of it before, I highly suggest you look into that as its pretty cool.
As for myself, I've never known any personally before they got patched. I always find out about them later on down the road when tons of other people have found & abused the shit out of them, and Jagex subsequently patches them and makes them disables the glitch lol.