My best prank: A year ago I was at my friends flat, which he finally rented with his new girlfriend. He is a kind of person, which has like constantly opened 110 tabs in chrome, three or four instances of chrome running, torrenting at full speed and in the meanwhile a few films having opened to "watch" later. He is very very secure about his computer and NEVER leaves me or anyone else alone with his computer. That day we were just talking in the same room, and he goes for some food. I was like yeah thats my chance to prank him. So I opened a new tab and came with an idea - what If I change his desktop background to some random chick, to prank both him and his gf. I knew she will not be mad but his reaction would be priceless (it was his first gf). So I started googling, found a three pretty naked chicks. This was like soft porn, they were still "dressed" but not much. I did not wanted to use a porn for this.
So I was about to download image - right click - save as - little window opened and..

...what the hell, that guy had literally like terabytes of porn in download folder, all totally in one chaos, thousands of images, millions of downloaded videos, all categories just everything from gangbang to milfs or old/ young, what the fuck that computer was like cursed station of porn.

In that point I was like fuck that. This prank has no sense then. So I just closed that little window and did nothing. Prank failed.

Nowdays, He still does not know what I know about his "hobbies". And I will never say him lol. About a months after he broke with his gf and moved to different house. He has now three monitors attached to his computer and 4tb of space. He is still complaining about "lack of space" and "too big downloaded movies" but we all know what is going on lol. We call his "working deck" a sacred porn station.