Well, was it a Silver Maple Leaf? Those have a $5 face value, but contain an ounce of silver, which is worth a little over $20 these days. Hopefully a bank teller would have known that. Or maybe this happened when an ounce of silver was worth about $10 (not that long ago.)
Edit: I should add that the bank teller would still not give you more than $5 for it, but a coin shop or a collector would pay you for the value of the silver.