Continuity mistake: When Peter is at the bar in Hawaii ordering numerous fruity drinks, he keeps taking off the fruit and piling it on the bar. A whole banana appears on the pile before he receives the drink with the banana. When he receives the drink with the banana, the pile of fruit is there but the banana is added. Then each time the shot changes, the fruit changes, with and without the banana.
Continuity mistake: When the hotel worker is speaking to Aldous about his demo CD, Aldous sits with his whole body facing forward. After the hotel worker does his English accent, Aldous says, "That stupid English voice, was that me?" and he sits with his whole body facing the hotel worker. The hotel worker then says, "You're really going to like it," and Aldous sits facing away, before finally Aldous says, "I'll listen to it" with his feet on the ground, facing forward, before it immediately cuts to him saying, "When you've gone". He now sits facing the hotel inspector with his feet on the seat.
Revealing mistake: When they meet up for dinner (Aldous, Sarah, Peter and the hotel clerk), the restaurant host says he is sorry that they have to wait 5 minutes, and you can see that Sarah is obviously way shorter than Aldous. From the front, Sarah comes up to Aldous' eyes, but when they cut to the back, she is almost below his shoulders. Then when they start walking, she steps off a platform of some sort. They seemed to make her taller for just that frame, and then when they are walking throughout the movie, they don't seem to compensate for the difference.