Not a one liner, but...
In an adventure with an old group, I played a blind Gnome sorcerer who had minor clairvoyance to sense obstacles within a 4ft radius.
The party was walking through some thick overgrowth when the GM handed me a note that I had tripped and fallen behind a little and to not speak to the party. He then pointed out to the group that nobody was keeping an eye on my Gnome and told the ranger, who had been hacking away at the plants to clear a path, to roll a D20.
Ranger rolls a 1, followed by a 19.
Quickly followed by our GM putting on a look of horror.
GM - "The sounds of nature fall silent as you raise your sword from the entangled undergrowth, you glance down at your sword and notice the crimson red now running off the tip of the blade." (something around those lines)
Everyone suddenly went a little sheepish and looks at each other and then at me.
Ranger - "Oh God, oh God... please don't let that be the Gnome... I'm so sorry!"
Everyone looks at me patiently as I read a second note handed by the GM.
Me - "Thanks for waiting guys; say, has anyone seen my familiar?"
The look of relief on the group's faces was priceless.
The ranger never owned up to killing my familiar in that adventure.
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My favourite one liner in an adventure would have to be from our GM where one of the player's character was a master of the elements who had to pray to his Goddess to use his power.
Player - "I call upon you my Goddess, grant me your powers over the elements: Earth, Fire Wind and Water!"
GM - "Heart!" [short pause] "With your powers combined, I am Captain Planet!"
The whole group struggled to keep a straight face whenever he went to use his spells for the rest of the day after that.
