Revealing mistake: When the "real" Ralph is first shown at the Peace Corp office, he has an unattractive and nerdy appearance/voice. When arriving at Rosemary's party in the car, when saying "Amen to that", he suddenly looks fairly attractive and his voice sounds less nerdy. Then when saying "Good luck Hal", his voice becomes nerdy again.
Continuity mistake: In the scene with Hal and Mauricio in the bar, they both have a beer. In the shot where Hal asks if things are okay between Mauricio and his girlfriend, Hal takes a drink from his beer and Mauricio is holding his beer down by his chest. In the next shot Hal is suddenly not drinking his beer anymore and it is out of sight and now Mauricio's beer is by his mouth. (00:06:55)
Audio problem: In the restaurant, when Hal is standing at the hostess table asking about the booth before Mauricio calls on his cell phone, you can see the hostess' reflection in the small mirror or window next to her and the movements do not match. The mouth in the reflection opens to say yes before she does.