If the player hits Meryl, she will retaliate with a hard slap, knocking off health. Meryl can end up killing Snake through this, and when she does the Game Over screen will have her voice saying "This can't be!"
If the player does this in the caves portion of the game and the dogs are around, she will call for the dogs to attack Snake. If the player immediately hides in the cardboard box after attacking Meryl, the dogs will instead pee on the box Snake is hiding inside of.