You can't lift the damn thing.

I insert a lame result.

you get a result that is Lame.

I insert Darkmatter.

You get a matter that is dark.

I insert the boring term of switcharoo.

You recieve a glorious Zillyhoo, and weep in it's majesty.
I forget to insert anything.

The machine explodes due to lack of use.

I insert a Die [you know like playing dice?]

You get a 6. Three times (666). Resulting in the summoning of Satan.

I insert Satan.

You get a piece of fiction with the word Bible on it. I wonder what it is?

I insert SCIENCE!

You don't get the BP oil company.
I insert a duck swimming in oil that says, "I love chocolate!"

You get fried and dyed duck.
I insert my Calc 3 homework.

You... don't get it back. Probably should've given it to your teacher huh?

I insert Duh's calc instructor

He/she gets the homework and gives Duh her deserved grade, by pushing it through the coin slot.

I insert a mechanical arm to get him back out.

The vending machine ReArranges the metal arm unto itself and no longer accepts items, it takes them.

The vending machine takes my shorts.

The vending machine absorbs the television next to it and becomes an abomination.
The vending machine takes the roof.

Wait? You didn't insert anything, so you don't get anything.
I insert
NINJA'D.
Wait... You didn't insert anything either. So you don't get anything.
I insert a ninja.

Sadly, the AI of a newly sentient vending machine is somewhat lacking, so it's not quite sure what to do with a nice fresh roof.
I insert The vending machine takes a book about Asimov's Laws.
Edit: ♥♥♥♥ Sakiskid, I'm keeping mine.