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 was actually filmed with Red One cameras that were supplied by Otto Nemenz International.
When Constantin proposes to Miss Piggy, he asks if she would like "to become Mrs. Piggy... or rather, Mrs. The Frog?" "The" is Kermit's middle name (Kermit the Frog ), not part of a double-barreled surname (Kermit The-Frog).
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.
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.