Other mistake: Near the beginning of the book, an imp says 'Are you going to warp today, Runt, or will pink flowers grow out of my armpits?' But later on in the book No. 1 says 'Pink! We don't have a word for that colour in the demon commonspeak'.
Continuity mistake: In the chapter "Imprisoned", there is a quote about Ark Sool which implies he is a dwarf, as it references his 'bum-flap'. However, in "The Opal Deception", it says he is a gnome.
Other mistake: Throughout the book No. 1 thinks/talks about how he wants to eat a cooked meal. In Chapter 3, First Impression, the reader learns that No. 1 finds it repulsive that imps "delight in chomping their food down before it stopped wriggling". Later in the chapter Abbot challenges the class to see if anyone can get through his armored plates. He personally commands No. 1 to try and get through them so No. 1 reluctantly uses a meat skewer that he had pulled from the fire. It doesn't make sense for there to be a meat skewer if demons don't eat cooked meals.