Calvin and Hobbes: Attack of the Deranged Mutant Killer Monster Snow Goons - Calvin's Mismatched Mittens
On page 91, Calvin was making a mini-snowman of himself to trick Hobbes from being pounced. But when Calvin goes down the porch to hide, his right mitten on his hand was white not gray. Calvin was supposed to have two gray mittens. Then on the next panel, the mitten turns back to gray.