At the anniversary party for the hotel Drac was not able freeze Mavis because "she's a vampire". At the end of the first movie Drac was able to freeze Mavis along with the rest of the gang during the 'Zing' song at the party.
Drac was able to hypnotize Dennis who is a vampire.
At 1'10", the window is closed then open between shots.
When they go to South America, the maps clearly show that they are in Brazil, but all of the signs are in Spanish and the people on the bus speak and sing in Spanish. In Brazil, they speak Portuguese.
When Drac and Johnny were on top of the mountainous cliff, they were seen smaller when they were falling down the cliff in the same shot afterwards.
When Drac is chasing Johnny after he changes into a monster, he hypnotizes Winnie and Dennis so they don't reveal to Mavis that Johnny is a monster yet, it is established in Hotel Transylvania 2 (2015), when Dennis is born, that vampires cannot be hypnotized by other vampires. However, it is possible that, as Dennis is half-human, that hypnotism would work on him, given the fact that the vampire side only comes from his mother Mavis.
Mavis finds large pieces of cloth that have been torn from Johnny's shirt , but when she finds him , the only part of his clothing that is ripped are near his shoulder .
There is a whole litter of wolf pups on board the blimp , 4 in the boat when it goes over the waterfall and 3 that Wayne uses to plug up the hole that was caused by the lava .
The number of wolf pups constantly is constantly changing from a whole litter , to 4 , to 3 and back to a whole litter .
Drac redoes the ice sculpture and has it showing it's teeth while smiling , but a little while later when it shows the ice sculpture melting , he is smiling with his mouth closed .
Johnny starts out with a backpack when he and Drac are hiking through the jungle , but somewhere along the way his backpack vanishes .
When Murray becomes human , he goes from having a mouth full of teeth , to having no teeth , to having about 3 or 4 teeth .
Mavis comes through a door just as Ericka gets off the elevator , how is it that neither of them saw Murray , Frank and the others before they dove behind the luggage dolly , as they were standing in plain view for a few seconds , long enough for both Mavis and Ericka to have seen them .
The ice sculpture is first shown with the eyes opened wide and a silly grin on it's face , when Drac redoes it , he makes it with both the eyes and mouth half opened , but a little while later when it shows the ice sculpture melting , the mouth and eyes are closed .
While walking through the jungle , a bird poops on Drac's hat , but in the next shot just before it starts to rain , the bird poop is gone .
The water in the pool is clear and blue , but in the first two movies , the water is a greenish color .
Drac and Mavis are shown singing the song "Just the two of us" over 125 years ago despite it being a little over 40 years old.
Despite it being confirmed in the first movie that Mavis, like her father, cannot go out in the sun, she happily travels through the jungle in the middle of the day without any ill effects.
If Dennis already asked his dad if that was him when he had turned into a monster and Johnny said yes , why then would Dennis need to ask Drac the same question when he already got his answer .