I remember playing Return to Zork with a friend of mine and we were cracking up about this moment:

We'll still quote it from time to time

Also there was another moment in Return to Zork when you say the wrong things to this person and they will finally punch your lights out.

Another time we were playing the point & click adventure Stephen King’s The Dark Half and we found the .ini files with all the dialog from the game and we changed everyone's name to something funny and added many curse words as adjectives. We actually started the game over from the start and was cracking up all the way through.