I remember GMing a D&D campaign once. 3.5e, good times. We played over the internet, with Maptools.
The players went into this ruined city, and went into the mage guild. The aboveground building was destroyed (by the dragon that destroyed the rest of the town), but the belowground stuff is all intact, so they head downstairs. So I describe the room for them: doors here and here, floors made of this stone, some flavour-text hanging from the walls, etc. And a big, person-size cauldron in the corner, filled with lots of magic.
FRIGGIN' LOTS of random magic, i.e. pretty much every spell that can be cast by someone in the mage guild. There's so much magic in this pot that the group's wizard had to roll a will save against damage when he used Detect Magic on it. So the Sorcerer...
Sorcerer: "I'm going to bottle some of it up!"
GM: "Okay. I think you've used a potion in the last fight, so you have a bottle... Uh, roll a Dex check to scoop it up without getting any on yourself."
Sorcerer: *rolls* "I got... uh, three."
GM: "Well... dammit. I don't have a table for something like this. I suppose I could just count the number of entries in the Wizard Spell List in the SRD and roll on that, but that's gonna be kinda pedestrian... gimme a sec."
So I went on the internet, and Googled something to the effect of "table of random effects" and found one with a thousand entries on it. I read the first five or so, looked somewhat interesting without anything too out-of-universe (i.e. nothing involving, say, spaceships), so I rolled a digital d1000 and...
GM: *rolls* ...Oh my god.
Players: "What? What happened?"
GM: "He... Turns into a Tonka truck."
Players:
Sorcerer: "...Crap. Can I still cast spells?"
GM: "Strictly speaking yes, but you can't use somatic or semantic components, so most of your spells are uncastable."
Sorcerer: "Can I still use Supernatural Abilities granted by feats?"
GM: "Um..." *checks* "Yes. But which- oh. Oh gods "
Sorcerer: "YES! I GET TO BE A FIREBREATHING TONKA TRUCK!"
I think he actually got a couple kills in that state, too.