You can buy pennies or nickels in bulk, melt them down, and turn a tidy profit. That's literally a "money printing glitch"

Every penny or nickel that gets lost in the gutter, stored in a coin collection, used as a washer or shim, turned into jewelery, or sets around in jars unused, represents a financial loss to the Treasury.

It's called seigniorage and melt value. Also see Gresham's law (hoarding).