On my last few paychecks I have recieved a $100 bonus. No mention has been made of this and my company is not the type to do this. My question is if they realize this is happening and its an error on their part can they demand I pay it back?

Absolutely!

If you were overcharged at a department store, you'd take back your receipt for a refund. The same logic is applied here. The thing to remember is that everybody makes mistakes. It's only human nature.

If you truly believe that this is an error, it's probably best to alert your boss now instead of later. Who knows if you'll have the money to repay when the error is uncovered by your employer.

Absolutely!

If you were overcharged at a department store, you'd take back your receipt for a refund. The same logic is applied here. The thing to remember is that everybody makes mistakes. It's only human nature.

If you truly believe that this is an error, it's probably best to alert your boss now instead of later. Who knows if you'll have the money to repay when the error is uncovered by your employer.
I agree, and you may even find that you were entitled to that bonus as a new incentive, you won't know until you check on it. Good luck!