When the four friends are in a French restaurant and Scotty, Cooper, and Jenny are trying to persuade Jamie to join them on their quest to Germany, Scotty's line is heard as "I Love Denmark," but his mouth form the words "I Love Amster..." as if he was in the process of saying Amsterdam before the shot changed.
Crew or equipment visible
Errors in geography
Incorrectly regarded as goofs
When tourists arrived to Bratislava (the capital of Slovakia), the town looked like a zone of military operations or armed conflict. There was neither war nor conflict in Slovakia, it's a peaceful and not at all a poor country. However, this movie is a parody of how Americans view different parts of Europe. Nothing in this movie comes close to accurately representing real European locations. This was an attempt at humor, not an attempt to accurately represent Slovakia.
When they get to England it is 10.40 am, which would make it the middle of the night in the United States. However, depending on deadlines, it would not be at all unusual for a partner at a law firm to request files in the middle of the night, and certainly not unusual for a partner to be in the office by 5:40 am (assuming the characters live in the Eastern Time Zone and Eastern Time is currently five hours behind London).
When their flight home from Rome is called, they are still in the check-in area. For an international flight they should have checked in, cleared customs, and passed through security before their flight was called. In a busy airport like the one in Rome, they will be lucky if they make it to the gate in time to board the flight.
At the beginning of the film, there is a map of Europe with most of the countries coloured in with their flags. However, Ireland is given the Union Flag (the flag of the United Kingdom), when in fact only the most northern part (Northern Ireland) is part of the United Kingdom. The rest is a separate country and should have been coloured in Irish colours.
Jamie gets mugged in Amsterdam and Jenny mentions that all their passports were stolen. While it is now possible to travel between the Netherlands, Slovakia, Germany, Italy without any passport controls, this was not possible in 2004. Slovakia ratified the Schengen Agreement in 2004 but it wasn't in effect until 2007 (for the other countries it was actually possible).