In describing the monk, the tattoo artist points out a design on the monk's arm. The tattoo artist says that the monk is from the "Chang Mai monastery, just outside of" Bangkok. Chang Mai is roughly seven hundred kilometers from Bangkok.
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.
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 ocher or orange in color.
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 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 may 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.).
When Alan is playing Ms. Pac-Man, right before the power goes out and we see the level he's on, 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.
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, & Flexeril) and ADHD medication (most commonly Adderall, Dexedrin, Ritalin, & Vyvanse) would have an amnesiac effect. The muscle relaxers alone by have made them loopy and pass out early, but not cause amnesia. And the amphetamine-based ADHD medication would not only have prevented any amnesiac effect, but they would've been so hyper-stimulated that they would've been in control of their faculties.
The goof item below may give away important plot points.
In the closing credits, Teddy chopped off his own finger while playing "Five Finger Fillet." In the shots before and after the accident, his Stanford ring is on the finger, but in the two shots showing the knife still embedded in the finger, the ring is missing.