The plane taking off from California seems to be a 747 by the shape of the nose, the one landing was an Airbus as previously stated.
When Alan is at the hospital eating a watermelon, there are dribble stains on his lap. When he goes outside, his pants are dry.
When Phil, Doug, and Stu are seated in the IHOP near the start of the movie, the amount of orange juice in Phil's glass changes between shots, though Phil doesn't drink from it.
When Alan is playing Ms. Pac-Man (1982) and the power goes out,the ceiling fans continue to spin for a while then stop then start again between shots.
When the Russian gives a piece of banana as a reward to the drug-dealing monkey, the banana skin is removed. However when shown eating the banana on the balcony, the banana skin is attached.
When Alan reveals how he inadvertently drugged his friends again, there is absolutely no way that a combination of muscle relaxers (most commonly Baclofen, Soma, and Flexeril) and ADHD medication (most commonly Adderall, Dexedrin, Ritalin, and Vyvanse) would have an amnesiac effect. The muscle relaxers alone could have made them loopy and pass out early, but wouldn't cause amnesia. And the amphetamine-based ADHD medication would not only have prevented any amnesiac effect, but also they would've been so hyper-stimulated that they would've been in control of their faculties.
When Alan is playing Ms. Pac-Man (1982), right before the power goes out and the level he's on is shown, an arrow from a mouse can be seen in the upper right corner of the screen. Arcade games are not equipped with this sort of interface.
In America it is not possible to update someone else's passport without their knowledge or cooperation.
When Alan puts a cigarette in the monkey's mouth shortly before leaving him it's visibly a hollow/fake cigarette before he lights it.
The Delta plane that is shown landing in Thailand is an Airbus A300. Delta has never had the A300 in their fleet, only the A310, which was retired in the mid-1990s.
When the crew arrives in Bangkok in the white cars, the stage light is visible in their reflections.
The protagonists travel to the wedding resort at Krabi on Thailand's west coast from Bangkok by speedboat in a relatively short time. To travel from Bangkok to Krabi by sea would involve a 1,500-mile journey through the straits of Malacca via Singapore and would have taken them at least 48 hours non stop, with additional fuel on board.
When Phil wakes up in the sleazy hotel, a Madagascar Hissing Cockroach is seen walking on the television. This particular roach is native to the island of Madagascar. Bangkok is home to Asian and Oriental cockroaches.
The Buddhist monks shown in the film are dressed in maroon/yellow robes, which are actually worn by Buddhist monks in Tibet. In Thailand, Buddhist monks wear robes which are either ochre or orange in color.
The plane to Bangkok is a 4-engine Delta Airlines 747, which is consistent with the announcement made at the airport that they were boarding a direct flight to Bangkok. Firstly, Delta does not fly direct from LAX to BKK. Secondly the plane seen landing is a much smaller two engine plane. It has been argued that the wedding is on an island and they could have taken an internal flight. However, the plane seen landing is also a Delta Airlines plane and any internal flight would have been on a Thai carrier. Therefore, there is an inconsistency between the Delta plane that takes off and lands.
When the guys arrive at LAX, Phil (Bradley Cooper) is looking at the departure table, which shows a flight to Copenhagen. There are no direct flights from Los Angeles to Copenhagen. One would need to either transfer at a major European city (like London, Amsterdam, or Hamburg) or an American city (like Chicago, New York City, or Washington D.C.).
Describing the monk, the tattoo artist points out a design on the monk's arm. He says that the monk is from the "Chang Mai monastery, just outside of" Bangkok. Chang Mai is roughly 700 kilometers from Bangkok.