Well, of course "THE" beholder in the underdark region. Buh!
But my personally most funny experience which hasn't been posted yet was with the eggs of the silver dragon. First Adalon wants you to get her eggs back, then Phaerie gives you fake eggs in order to see the matron mother getting killed when offering the false eggs to the demon. And finally Solaufein offered the next pair of fake eggs in order to fool Phaerie as well, that's where I fell of my chair laughing
Watch the dialogue what the demon says after he's been offered fake eggs two times in a row
Archimedes, the Greek, said 'All Grecians always lie'.
Later in the afternoon he corrected himself and corrected this:
'All Grecians always say the truth'
Where there's a will, there is a way.
And if there is no way, then we're going to hack and shoot it clear!