When the car carrying Porky (inside an open-ended garbage can) hits the curb, a dummy with Porky's shirt is used for the impact. The dummy fills less than half the garbage can (while the actual Porky kneeling inside the can filled most of it), and the bare dummy arm extending forward has no hand on it.
As Buckwheat and Porky's car speeds down the hill the right front wheel hits a rock, and Buckwheat's seat - as well as Buckwheat himself - separate from the rest of the car. Close examination reveals that the separation takes place before the wheel hits the rock.
When Butch and Woim crash, the wires pulling them up in the air are visible. In the next shot, the wires lowering them into the stream are visible.