Halfway down the back of the guy on the front cover there is a diagonal scratch that obliterates some of Robert Crumb's cross-hatching.
The scratch has been touched up by the addition of fine lines (thinner than Crumb's cross-hatching).
SRUCC: Zap Comix Series
Comixjoint: Zap Comix Series
Comixjoint: Zap Comix #0, 4th printing
Grand Comics Database: Zap Comix #0, 4th printing