Other mistake: In the auditorium where all the cartoon characters are assembled, there are two Sylvesters (the black and white cat who was always trying to get Tweety). There's one sitting in the front row, Tweety behind him, and another Sylvester behind Tweety. It's seen right after the blue alien says, "No foolin'."
Continuity mistake: As paramedics inflate Jordan's assistant Stan Podolak following his lone bucket, the scoreboard shows the Monstars ahead 77-67 with 10 seconds remaining. Yet following his treatment and the surprise entrance of Bill Murray, the score has changed to 77-76 with no time having elapsed.
Revealing mistake: When Stan falls into the dug-out at the baseball game you can clearly see the crash mat that he lands on.
Character mistake: When Stan inflates himself, Michael asks, "How'd he do that?", implying that he doesn't know the rules of the Looney Tunes world. Since Michael was rolled into a ball by the aliens earlier in the film, this shouldn't be too much of a surprise.
Deliberate mistake: When the Looney Tunes are playing basketball against the aliens, if you look in the stands the same cartoon characters are in different places at the same time. This happens a lot.
Continuity mistake: When Stan's body has been "deflated", his body is laying over the 3-point line. Then when the doctors rush over to him, he is laying a good distance away from the 3-point line. When the air pump is stuck in his mouth in the very next shot, he is laying over the 3-point line again.
Continuity mistake: The aliens' drawbridge drops on Elmer Fudd's head then the aliens come down it. When you see the drawbridge again Elmer Fudd has magically disappeared.
Continuity mistake: When Tweety is surrounded by the Monsters and he fights them off with karate, the timer is visible in the background, set at 12:00. Before this scene, the timer was at 5:01.
Continuity mistake: When Bugs Bunny and Lola Bunny are playing one-on-one, there is a shot of Lola entering the free throw lane. Then in the next shot as she starts dribbling, she is suddenly standing outside the free throw lane, and then enters it again.
Continuity mistake: When the alien boss is watching Looney Toon cartoons in the beginning, the cartoons change between shots of him saying "Looney. That's the word I was looking for" even though they are not changing in earlier shots.
Continuity mistake: After the Looney tunes challenge the aliens to a basketball game, the aliens change position when they all yell "All right, basketball.".
Continuity mistake: After Wile E. Coyote crashes into the auditorium, Daffy steps on his nose. On the very next shot, Daffy is walking in the aisle, but Wile E. is not visible behind him.
Other mistake: The Phoenix Suns/New York Knicks game (with the Knicks as the home team) is being played at the Los Angeles Sports Arena.
Continuity mistake: When the orange Monstar crushes Bugs Bunny after he pushes Lola out of the way, Bugs is lying face down, but after the Monstar gets off him, Bugs is now facing upwards.