Continuity mistake: When Spike sees Droopy picking up his wallet containing the dynamite, Spike runs off and you can see he is holding a cane in his right hand, but when Spike jumps on the cow in the next shot, the cane has gone.
Continuity mistake: When Spike sets the shack on fire, he opens the door and you can see there is a knob on it, but when Droopy opens the door and rescues the lady from the fire, the knob has gone. The knob reappears when Spike goes back into the shack after Droopy rescued the lady.
Continuity mistake: The flagpole on the ledge of the skyscraper with Spike's milk bottle on it was not there originally when Droopy rescued Spike.