Revealing mistake: In the last scene of the episode, Angel kicks the guy out of the glass building. This guy's a vampire, and you can see his reflection in the glass windows. Vampires aren't supposed to have reflections.
Continuity mistake: The package Nigel holds in his arms near the beginning of the episode completely changes between shots.
Other mistake: When Veemon, Armadimon and Hawkmon are about to rush in and save Gatomon, a previous frame of Hawkmon's beak stays on-screen. (00:16:15)