Right after the hunter salts the celery stalk, Bugs's feet flash gray, indicating a mistake in coloring the cell.
When the bunny is stamping pretend footprints on the ground, one footprint next to the big white rock is already on the ground without being stamped there.
The human at the start of the cartoon grabs his double barrel shotgun. When he shoots at Bugs Bunny, he fires his shotgun in quick succession 7 or 8 times without reloading. He would have only been able to fire twice and then would have had to reload in order to have shot twice more, and so on. As there were self loading semi automatic shotguns at the time, the cartoonist could have easily this shotgun one barrel (and inferred that it was such a self loading semi automatic shotgun). Then it would not have been very conceivable to have had the hunter fire 7 to 8 times without reloading.