When a clad coin has been in a fired, there can be a bubble that shows on the coin:

Often these are flattened as they will not work in a vending machine. But there is no premium for these, they are just post strike damage. But always knowing what happened is always better than wondering what happened to the coin. Now you know.
CoopHome:
What causes a bubble on clad coins? usually the coin is heat, like in a house fire or a campfire. Just a damaged coin now. Still spendable though. (face value now only)