One KFC here won't take 100s and have a sign posted saying it in advance.  The other ones don't.  I got stuck at a grocery store looking for a quarter to unlock one of their carts and only had plastic or a hundred.  Felt funny changing a hundred for a lousy quarter for a cart but at least they did it. One Toronto corner store I was in checked every bill that came across the counter. They said that counterfeit fives were common because so few people checked them.

As far as the original poster goes, they probably flagged the purchase as unusual and denied it for "your" protection.  Which might be THEIR protection, depending on how liable they are for fraudulent purchases that they might be able to anticipate. Happens all the time with credit cards.  I bought enough parts to build a computer once.  That transaction went through but the card got locked after that and my wife got stuck not being able to buy something because the joint card got declined. Pain in the butt but at least they're trying to minimize damage from crooks. It kinda took the shine off of a surprise Christmas computer present though when she called the credit card company to find out why she got declined.