The roles of John Banner and Werner Klemperer were miscast
The actor who played Werner Klemperer has popped up in a lot of movies and he's a good, versatile actor - even in this. His frame seemed too burly for this role in my opinion, however. Klemperer needed to be played by a slighter individual.
The actor who portrayed John Banner played him as a creepy looking, John Wayne Gacy type rather than the loveable teddy bear that John Banner came across as.
Just my two cents.