You could collect hobby recognized variations, recurring print defects, one time print defects, out of registrations, wrong backs ( or fronts
), blank backs, proofs, miscuts , or just some of the above. Sometime back I chose to stick to recognized variations and recurring print defects on the theory there has to be some limits
I still wonder if one searched long enough you could find a variant for any pre 1981 post war card ( The only pre war set I collect also has a variation for each card in the set
)