My oldest error coin is probably one of my medieval islamic coins.
These are generally either off-centered, weakly struck or double struck or a combination of that.
But on these coins it is not a quality and you will be looking for the one that is well centered and fully struck.
So I will show a pair of French Revolutionary Décimes struck in the Year 5 of the Revolution which equals 1796.
I got this spectacular off center struck which is still fully identifiable for it's type, year and mint mark.
A couple of years later I came across this one (same type, date and mint mark) which is similarily off center struck with the incuse reverse on the other side.