A few...
One of my colleagues (and he *is* a reader here) left an interesting error message in a site he developed. The message was an in joke around the team, and was "Get back, thickyhead".

The first I heard was when a user came to me asking why one of our website was telling him to get back, and calling him thickyhead.

My colleague had put the message in as a joke and forgotten about it. The code was designed so that a lot of things had to occur before it was displayed. So, it took months for it to appear.

The message as quickly removed when I asked about it

As for my favourite, well, two. They both happened to the same user who was, shall we say, a little eccentric (to say the least).

One morning, he came in, wearing his headphones (big, over the ear "cans") and asked me a question. When I replied, he just shouted "What?". I politely (but loudly) asked him to remove his headphones, to which he shouted "What?". My boss appear behind me and shouted "Take off your damn headphones". At which point the user did, and could hear me..

The other occasion, the user came into the office, and was just about to launch into a tirade about something, when my boss looked at him, and said firmly, but politely, "Before you start your question, do you think you could tell me the best way to London City Airport?". The user was so surprised that he answered and left the office immediately.
(Ok, I'll get me coat, Wed 30 Sep 2009, 16:25, 5 replies)

now
this i like
(Roosta, Wed 30 Sep 2009, 16:37, closed)

Been there...
A site I made for a business years ago had an error message left over from the testing phase.

Basically, if you take into account that the URL format would be something like this:

www.somerandomwebsitethatimade.co.uk/document.php?id=123

If the number at the end was invalid (due to someone pissing around, or a deleted document that had been previously linked to) then it brought up the rather gracious error message "You've buggered it up, you knob!"

(on a similar subject, the 404 error page for my team fishcake website used to have massive text reading "YOU STUPID FUCKING CUNT")
( Mike Fishcake teamfishcake.co.uk - better than your mum's face, Wed 30 Sep 2009, 16:40, closed)

...
lp0 is on fire.
(haberman is singing in the changes, Thu 1 Oct 2009, 12:20, closed)

I think
every developer has done this at one time or another. I've had users ring up asking what an "Out of Badgers" error was, I almost feel like asking them whether they have any badgers or not.
(SpangoTheWonderBadger is brewing beer, Wed 30 Sep 2009, 16:43, closed)

All these "shouldn't see this" type error messages...
... are hilarious :)

Rule 1 of programming: no matter how unlikely you think it is for something to happen, the user will find a way. None of these hidden error messages are a wise idea. (They are quite often amusing enough that they understand and laugh when you explain, unfortunately some bosses don't share the "purple donkey error" sense of humour)
(Cloudane, Thu 1 Oct 2009, 1:03, closed)