Mad Hatter(s)

The Mad Hatter first appeared in Batman #49 (October-November 1948) in a story that also introduced Vicki Vale.






But the next time Batman and Robin encountered a "Mad Hatter", he didn't look anything like the one from back in 1948.
From Detective Comics #230 (April 1956):




This was the version of the Mad Hatter that was in new Batman stories thru the 1970s.

But then there came Detective Comics #510 (January 1982):




and the explanation given in that story as to what was going on here regarding "Jervis Tetch",



As for how they presented all this in the Mad Hatter's Who's Who entry in Who's Who: The Definitive Directory of the DC Universe Vol. XIV (April 1986):

The Jervis Tetch imposter eventually turned up alive years later, in Batman #700 in 2010.