When Christopher Robin visits London Zoo (in 1928), he watches penguins swimming past a viewing dome set into the side of their enclosure, known as Penguin Beach. This wasn't built until 2011. In the director-screenwriter DVD commentary at 0:28:24 director Simon Curtis mentions "And we cut to these shots where it's actually the real London Zoo at Regent's Park. And this window, this circular window is apparently accurate to the interwar years." An underwater, flat, circular window may have predated World War II, even though an underwater, three dimensional, spherical window did not.
When the Guards band is playing for Christopher Robin's birthday, they mark time out of step with the music (the first beat of the bar should be on the left foot). The apparent error probably arose in the editing stage.