Going through my nickels, I notice something on the reverse that I hadn't noticed before. I'm assuming it is an improperly mixed alloy planchet (woodie) but wanted to share and get others opinions.
I've only found them on one side on many cents ( so far ) but have yet to find a nickel, so not really sure.
Improper metal mixture of the original long planchet strip. ...and yes you can find them on both sides, but it's generally one side of the coin.