What's up with this coin?

I recently found a strange quarter. It’s lighter than usual, and the reeding is uneven (in some places, the edge is almost entirely copper, in others, almost entirely nickel, but both metals are on all parts of the edge). I can’t find any info on a coin error of this type. What am I looking at here? My mind jumps to "counterfeit," but I’d like a more experienced opinion.
1999 Pennsylvania State Quarter
Philadelphia Mint
seems otherwise normal for a circulated coin of this age