During the scene where Al hits his thumb with the hammer, you can see that the hammer actually hit the wall several inches away from any part of his hand.
Al hits his left thumb with the hammer in his right hand. In the next scene Peg is sitting next to him bandaging his right thumb. His left thumb is bandaged too, which happened when he was baiting the mousetrap, which a right handed person would do with his right hand.
Al: What do I say, Peg? I say stand back, 'cause I'm kicking some mouse ass!
(Al is holding a piece of cheese out by a hole while holding a hammer)
Bud: Mom, even if the mouse is stupid enough to stick his head out for the cheese, won't Dad smash his hand with the hammer?
Peg: Yes, Bud, he will. But, you know, everyone who watches Roadrunner cartoons knows the coyote is going over the cliff, but you watch it and laugh anyway.
(Al sees something and accidentally hits his thumb with the hammer)
Peg: Beep Beep!
Peggy: Al, it was horrible, it was terrible, I, I've never been so scared in my entire life.
Al: What's the matter, you see the vacuum?
Bud: Hey, did anybody here any noises last night?
Peggy: Well, your father had gas.
Bud: No, these were noises I never heard before.
Kelly: Well, then it could have been a girl, moaning your name.
Peggy: Is he down there? Did you get him? What happened, Al?
Al: He doodied on the trap. And in my bowling shoes.
Peggy: Oh, my god! He was in your shoes and he lived? This is no ordinary mouse.
(Al is going into the basement)
Al: Say, cheese. Hello!
(Al fires twice with his rifle)
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