Continuity mistake: When Sylvester throws the baseball containing Speedy back into the cheese factory, the ball stops in front a crate. But when we see Speedy emerging from the baseball in the next shot, there are now two crates behind the ball.
Continuity mistake: When the crowd of mice await Speedy's arrival, you can the egg box used as the "El Cheese Bowl" has "EGGS GRADE A" on the front. But when Speedy returns with a piece of cheese, the front of the box now says "EGGS."
Continuity mistake: When we see the four mice at the start of the cartoon, you can see there is nothing behind them, but when the camera pans in on them in the next shot, a cactus and a huge rock appear from nowhere.
Continuity mistake: When Speedy is bowing to the other mice when he comes back, you can there are four Sombreros behind him, but after a quick cut to Sylvester, we return to Speedy and a flower appears from nowhere.