During the "I Can Give You What You Want" musical number, Constantine holds hands with Miss Piggy as they spin around: Constantine is spinning to his left, thus Piggy should also be spinning to her left, yet she spins to her right.
The CIA is not a law enforcement agency, so Sam the Eagle has no claim to jurisdiction at the gallery in Berlin; the CIA's mandate is solely the gathering and analysis of intelligence. And as an operative of a clandestine organization, Sam would not carry anything that identified him as CIA; he would have a cover identity, which, as America's primary diplomatic arm, would most likely be as some sort of representative from the Department of State.
When the train gets to Berlin, the caption says "Hauptbahnhof station". Since "Hauptbahnhof" already means central station, the caption actually says "central station station", i.e. the added "station" is totally unnecessary. "Berlin central station" would have been perfectly applicable to name the station where the Muppets arrive.
The sign in front of the (fictional) national treasure museum in Berlin contains several mistakes. It reads "Nationale Schatz Museum", which seems to be translated literally. First of all, national treasure would correctly translate to "Nationales Kulturgut" (national cultural asset). In combination with museum (same word in German), the order would change to "Museum des nationalen Kulturguts" (in order to avoid the ugly dash construction "Nationales-Kulturgut-Museum", which would rise ambiguities since it is not clear whether the Kulturgut or the Museum is national). Further, the grammar of "Nationale" is wrong. One would say "Das nationale Schatzmuseum" but since you wouldn't use an article with that, it would be reduced to "Nationales Schatzmuseum" (mind the 's' at the end of "Nationales"). In this form, it would be the museum that is national, not the "Schatz" (treasure).
To sum it up: The sign is a very sloppy try to imitate a real name and is supposedly unintended funny for German-speaking viewers.
in the opening scene, when the production crew wraps up filming the closing number of the first film, a Panavision Panaflex camera can be seen. The Muppets (2011) was actually filmed with Red One cameras that were supplied by Otto Nemenz International.
Someone mentions American President Bill Clinton, whose term began in 1993. Communism fell in Russia in 1991 but the gulag guards are still wearing Communist uniforms (look for the red stars bearing Hammer and Sickle).
When Dominic is suspended in the room with the Crown Jewels, none of his ropes trigger the laser alarms.
At about an hour and 2 minutes into the movie, there is a brief view of the Moon. The Moon is shown reversed rather than the way it should be when looked at with the unaided eye.
When Sam Eagle and Jean-Pierre Napoleon met for the first time, the caption states "Interpol headquarters, Geneva, Switzerland." Interpol HQ is in fact located in Lyons, France.
In the mirror scene, Kermit's and Constantine's boutonnieres are not "reflections." Both characters have them on their left lapels.