The guard riddle in the movie Labyrinth (with David Bowie) reminds me of the everlasting battle between IBM and Microsoft, which was brought up once again during the last LotusSphere 2008.

Solve the riddle, and you will know the absolute truth between the claims of IBM and Microsoft.

SPOILER WARNING! Don't watch the video before you have solved the riddle yourself.

Here goes the riddle dialog:
There are 2 doors. Sarah needs to find out which door leads to the castle at the center of the labyrinth, as the other one leads to certain death.
Guard: "You can't ask us. You can only ask one of us."
Guard: "It's in the rules, and I should warn you that one of us always tells the truth, and one of us always lies. That's a rule too." Gesturing to the Guard, "He always lies!"
Guard: "I do not! I tell the truth!"
Guard: "Oh, what a lie!"
Sarah: "Alright," to the Guard, "answer yes or no: would he (Guard) tell me that this door leads to the castle?"
Guard: "Uhhh...yyyes?"
Sarah: "Then, the other door leads to the castle, and this door leads to certain death."
Guards: "oooOOoooh."
Guard: "How do you know? He could be telling the truth!"
Sarah: "But then he wouldn't be. So if you told me that he said 'yes', I know the answer is 'no'."
Guard: "But I could be telling the truth!"
Sarah: "But then he would be lying. So if you told me that he said 'yes', then I know the answer would still be 'no'"
Guard: "Wait a minute," to the Guard, " is that right?"
Guard: "I don't know – I've never understood it!"
Is Sarah right?

Spoiler Video: Labyrinth Riddle
Please note that there are also other hidden truths in the video...