I used to work at a place where a guy used, "Hello Fuckers" as test data. Our manager got a very angry call from a customer who printed out a 300 page report with, "Hello Fuckers" at the end of every line.
10 years ago I built an internal system for tickets (really old school VB and internal access db) for helpdesk & ISM the internal error code for closed tickets due to user error was id10t it was an internal joke until they decided to pull data from it into the new CA unicenter system which then lead to a bunch of quite important people's names appear with the column next to it saying id10t on the big 50" plasma screens in the new NOC/IT operations center.
I know I am not supposed to, but I laughed at this one :p
One of my former colleagues used to put weird error messages (made up words mostly, but really funny to pronounce). One of these messages escaped QA and went to a customer - but this was a nice guy, and he called up and asked what that word means (it was a while ago, can't remember the word now).
We didn't have anything to do one afternoon, so we searched google maps for names of funny places - there is a town called Hell. Imagine going there and reading the sign "Welcome to Hell". Good times were had at that job :p
I had idea to use something like this in our test data, but then decided to use "John Malkovich" instead. Indeed, it worked really good when test system accidentally sent data to prod system: everyone was immediately alerted, but nobody were offended.