Factual error: Milo says Audrey is a "teenager". Which she is, but this is 1914! People didn't use that word back then. He probably should have said "adolescent" or something.
Factual error: When Milo returns from his underwater investigation with Kida to find his companions armed, we see Mole holding an old-style submachine gun similar to the Russian PPSh-41. But it's 1914, and the first submachine gun - the German Bergmann - wasn't invented or to be introduced - until 1918.
Factual error: When Mole shows off his dirt collection under his pillow, each dirt pile has a flag sticking out. However, several are either incorrectly drawn or could not have been in use in 1914, the year in which the film is set. 1: The Welsh dragon flag was not officially adopted until 1959, and since Wales would not begin devolution in earnest until after the Second World War, it should not even be a separate entry. 2: The Nepalese flag used is the post-1962 version with differently-sized triangles - the flag in 1914 would have had triangles identically sized. 3: The Turkish flag used has a crescent that points diagonally towards the upper right, whilst the flags used by the Ottomans in 1914 had crescents which faced the right. 4: The yellow lozenge on the Brazilian flag should touch the centres of each edge, but is shrunk in the film's version. 5: When zoomed in, the flag of Poland can be seen. Poland did not even exist as a country in 1914, and would not be reconstituted until 1918. More distressingly, Mole seems to be missing several samples from countries neighbouring those seen which he is likely to have travelled through. For example, to get to Poland he would have had to have gone through Germany, Russia, or Austria-Hungary, between which Poland was split. To get to Wales he would have had to have gone through England. To get to Nepal he would have had to have gone through either China or the British Raj. Disturbingly, he has no French dirt in spite of being a Frenchman.