(at around 22 mins) When Simon attempts to catch the toaster waffles, two are seen flying through the air, but only one lands.
(at around 43 mins) You can see a poster on a record store with the chipmunks wearing their sweaters before Dave makes them.
(at around 30 mins) The "coloring" that the chipmunks did on Dave's presentation posters depicted Simon with glasses. But the drawings were done before it was discovered that he needed glasses.
(at around 1h 5 mins) When Ian takes Simon's glasses away and gives him new ones, Simon has the old glasses in the following montage.
(at around 10 mins) When Dave throws the muffins away, the garbage bag partly falls in. It is fixed in the next shot.
(at around 1h 20 mins) When Ian lowers the curtain during the concert he pulls the rope down. This curtain system actually requires the operator to push the rope upwards to lower the curtain (counterweight system).
(at around 54 mins) When Claire is taking pictures of the Chipmunks at the party you can see a flash go off directly at them, even though Claire's camera doesn't have a flash attached. However, the Chipmunks are being flashed at every few seconds throughout the scene, even before Claire comes to their table, and after she leaves as well. It's obvious that all those flashes come from the other cameras nearby, not from Claire's.
(at around 11 mins) When Alvin opens the cupboard, there is a bag of Utz brand cheese balls. Utz brand is only distributed on the east coast. However, many specialty stores in California carry Utz items that they special order and have shipped.
(at around 33 mins) When Dave runs out of his home to get dinner for his date with Claire the time is 6:30 pm but it is still daylight. The movie occurs shortly before Christmas and at that time of year it would be dark at 6:30 pm.
(at around 1h 16 mins) In the dressing room scene before their world tour concert. you can see Ian in the mirror, but when Alvin turns around, Ian is not in the reflection.
(at around 54 mins) When Claire is taking pictures of the Chipmunks at the party, you can see Dave's hand reflected in the lens, but not the Chipmunks.
(at around 23 mins) Dave takes the glasses from a Santa doll and puts them on Simon. It is unlikely that a Santa doll's glasses would have prescription lenses, much less prescription lenses that are the exact prescription Simon's eyes need.
When Dave roots through cabinets with a spatula spilling out food and dishes from inside, his belly can be seen from under his shirt.
(at around 1h 24 mins) As Dave's car drives away with the Chipmunks getting to drive, beep the horn and work the wipers, the sound for the wipers can be heard, but the wipers aren't visible through the windows.
(at around 1h 5 mins) When the Chipmunks go on tour and cities are listed, it says, "Phoenix, NV," not, "Phoenix, AZ," when it was clear from the map that they were in Phoenix, Arizona.
Even though the Chipmunks were talking loudly, Dave didn't hear their voices when they were trying to jump into the basket.
Despite not being able to afford the toaster waffles, Dave didn't take them out of the shopping cart.
(at around 56 mins) During the premiere party Simon says if he continues to eat he will go into insulin shock. Insulin shock occurs when a diabetic does not eat for an extended period of time. Given that there is never any other indication that Simon is diabetic (in fact he'd never survive in the wild if he was), he probably said it simply because it "sounds cool", without really understanding what he was saying.
(at around 1h 4 mins) Ian says the Chipmunks are going on a tour "coast to coast in 5 days". However, the schedule shown during the montage shows at least six cities, starting with Los Angeles, CA - July 4th, and ending with Austin, TX - July 28th.
(at around 55 mins) Ian tells Dave the Alvin doll speaks Spanish, but it is speaking Japanese. The doll says "Konbanwa. Watashi wa Alvin desu." which means "Good evening, I am Alvin".
Simon says that chipmunk parent's take care of them for a week before taking off. in real life however, the young stay with their parents for two months before they begin to gather their own provisions for the winter ahead.