Revealing mistake: When Ace and Melissa are in bed together having sex if you look close enough you can see that she is wearing underwear. (00:45:35)
Revealing mistake: At the end of the movie, when Ace is at the stadium and he attacks the mascot, his name is called and he looks up to smile and wave. He is surrounded by players from the Miami dolphins, but if you look through the crowd of players you can see the stadium seats are empty. (01:18:25)
Revealing mistake: In the scene where Ace visits Woodstock's secret office at the music club, pay attention to the computer screen when Ace asks him to tap into the aquatic supply stores to track the sale of equipment for transporting a dolphin. Woodstock says, "All right, we got marine winch, sling, feeder fish, 20,000 gallon tank..." As he is saying this, the computer screen shows a bunch of non-related items (plates, cups, concrete, stone) with crazy numbers that don't make sense. It was as if someone was in a hurry to create a fake spreadsheet to use in the movie that they just pounded on the keyboard and a bunch of numbers came out. If you look through all of the items, it's actually funny how they are so varied (dog food, speakers, sneakers, soup, chairs) and have nothing to do with the story. Many of the items also repeat with different sets of numbers.
Revealing mistake: At the very beginning, when the guy is hanging on to Ace's car and Ace drives off, there is a shot of the dog biting the guy, but if you look closely, you'll see that the dog doesn't even open its mouth. It just sort of brushes his hand. (00:04:55)
Revealing mistake: When Ace Ventura is checking out the tank at Ronald Camps party (looking for Snowflake), as he's holding the fish over the water, whistling and saying "Here Snowflake", you can see the tip of the sharks nose, waiting to jump out of the water. (00:29:40)
Revealing mistake: When Ace's car flips over you can see the left wheel is elevated over the bush. If you look closely there is a wooden plank there to help the car lift itself over the bush. The other mistake in the shot is that there is no conceivable way the bush would have flipped the car. Unless there was something very solid in front of the bush, the car would have just gone right through the bush. (00:52:05)
Revealing mistake: After Ace loses the guys in the car chase who kidnap Dan Marino, Ace goes hurtling through the park in his car and smashes through a picnic table. Just before his car flips you can see his windshield is smashed but no holes through it. As the car flips over the bush there is a sizeable hole in the windshield on the drivers side. Its likely the hole was for the stunt driver to look through on the flip. Ace in his car has to keep his head out the window to see anything in front of him (because of the smashed windshield). On the flip the guy has his head in the car. Later on the hole has fixed itself. We see this as Ace is driving Dan Marino to the Superbowl the windshield is still smashed but no hole and throughout the film its made very clear Ace doesn't have the money to fix it up at any point. (00:52:05)
Revealing mistake: When the bad guys are shooting at Ace, he catches a bullet in his teeth. However, he goes from his lips being tightly shut to suddenly having the bullet in his teeth, obviously hiding the bullet in his mouth. (00:52:00)
Revealing mistake: When Ace is at the mental hospital and slams his face down on the bench, you can see the actor playing the doctor begin to laugh before the camera cuts away to a front shot of him and Courtney Cox.
Revealing mistake: When Ace and Courtney Cox are kissing in the stadium, you can see the cardboard cutouts in the upper deck (replacing live fans).
Chosen answer: That scene was cut originally by the director Tom Shadyack because the scene did not support the story and thought it slowed the film down. (And the scene was cut from the US version as well.) It was added back later in TV versions of the film. Fan enjoyed the scene so much they asked it be put back in the film version. I know the DVD has it because I own it. It's hilarious!