Machine doubling? Just a guess from seeing about a hundred threads similar to this. I spend all my error coins...
I'm sure they are... but I don't collect them or bother to search my change for them. So I guess whatever errors I've had in my possession have been spent.
I use to do the same thing until I found a 1971 D Kennedy Half on a Silver planchet in 1989. I sold it for $900.00. Now I take the time to look at all my change.