When we see Tweety singing, you can see that the bottom of his cage has nothing on it, but when we see Sylvester next to the cage, a door has appeared.
After Sylvester has swallowed the bowling ball, he goes back down the drainpipe and out into the street. When this happens, look behind him and you can see a sign saying "Drink Friz! Six Delicious Flavors" - a reference to the director Friz Freleng. There is also a building with "Champin's 5 and 10" on it - a reference to the animator Ken Champin.