Love the topic, and I have multiple.

We were designing a frontend for a new application and we were using University lingo for the text placeholders. I forgot to remove one section in which the text stated "You are looking to enroll in the University of Deez Nuts", on another section I left "Click here cuh". Our manager at the time liked the design so much, but forgot to check for spelling or texts and as such sent the demo to our entire department. Everyone saw it, and while they all found it funny it could have seriously gone wrong. Thankfully our department VP had a pretty good sense of humor.....dude also knew exactly who it was from the start.

On another application, a director, who is a friend, asked for multiple items on a request form, during testing, I added text in Spanish (I am in Texas, but Spanish is pretty well known and spoken in the state) saying "Que bien chinga <Name of the Director>" which roughly translates to "<Name of the Director> is being annoyong" (but in a very Mexican spanish way)
I neglected to consider that the dude was probably viewing the admin board and checking the items as they were being added to the system and he called me not even 3 minutes later saying "You know I can see what you add right??"

All in all, I was pretty lucky because in any other places I would have been severely reprimanded :P

There are many more, but these came at the top of my head as the better ones.

My friend is learning PHP and I told him to install xampp but that fucker went nuts and installed MySQL too which didn't allow the xampp's SQL and ended in conflict.
Oh man that's so funny when non programmers don't do what we say.