It's clear from the text that it is a mistake. He sees a male in the garb of Athens and, as that was the description Oberon gave him of the man he wanted squirted with the Love juice, Puck squirts him. He even explains that to Oberon later. Everything Puck does, he does honestly. When he makes mischief, he says so. He says so because, to him it is wonderful, and because he is, in his own words, which I believe are the truth, " And, as I am an honest Puck, . ."These words are in the final speech of the play, where all is open and masks are laid down for the night.