In the scene after Beethoven "attacks" the vet, Dad goes into the backyard and gets Beethoven to take him to the vet's office, the dog's neck and snout are mysteriously clean even though only moments earlier they were smeared with fake blood.
When Alice places bacon on George's plate, she puts down two. Before Beethoven eats one, there are three. This is because the same footage of bacon being placed on a plate was used twice (which had two pieces).
At different times, George drives a wood-paneled station wagon and a solid brown station wagon.
After Brie knocks over her glass at the barbecue, Alice picks it up and it is intact. When she walks away with it, it is broken.
When Ted drives the car into the warehouse, he runs into the table with the needles twice.
In real life, the weight of male St. Bernards would not average up to 185 pounds, especially not at 1 year old. According to the American Kennel Club, the average weight a male St. Bernard reaches is exactly 180 pounds. Also, at 1 year old, a male St. Bernard would only weigh around 150 pounds. However, as Beethoven is fed rather constantly, it is possible that he may just be 35 pounds overweight.
Beethoven's owners are seen feeding him bacon and Twinkies, and he eats other human foods like cookies when nobody's around. Such foods are known to give dogs indigestion, and too much bacon could give a dog pancreatitis.
One little girl is seen feeding Beethoven her vanilla ice cream cone as she continues to lick it herself. Understandable as she's just a child, but vanilla is likely not to sit well with Beethoven either. Had it been a chocolate cone, he'd be even worse for the wear.
When Ted and Rice are playing Super Mario Brothers 3, they are both mashing away on controllers; however, when it shows the close-up of the screen, we see that they're only playing a one-player game at the time.
When Beethoven dragged the table and George's obnoxious clients at the sidewalk down the street, the table had "skis" on it.
When George comes into the house during a rainstorm. You can see clear sunny skies outside.
When Harvey and Vernon hop the fence into the junkyard and the dobermans come out, a trainer or other crew member can be seen behind the pallets.