Plot hole: 1-6 "A Gummi By Any Other Name": The Changing Hat has the habit of tranforming its wearer at the mere mention of a name (or even a species type, as seen before when Sunni accidentally transforms into an ogre). However, when Calla impersonates Igthorn and faces off against him, she loudly says the name "Toadwart" and doesn't transform.
Factual error: 2-7 "The Crimsom Avenger": At Dunwyn Erwyllan grabs a bundle of money bills from a fishmonger. It may be just a curiosity since paper money wasn't used in real medevial times, but it is still odd as gold coins are still in heavy use in all other episodes. Even the royal treasury, which is robbed in this very episode, has only bags of coins and no bundles of paper money.
Continuity mistake: 1-6 "A Gummi By Any Other Name": First Cavin runs out of the prison tower, then the Gummis (Sunni, Tummi and Grammi), and then Igthorn, each of them smashing the door into the guard's face. But in the instant where the Gummis appear, Princess Calla is missing even though she is supposed to be with them.
Plot hole: 5-10 "Princess Trouble": The catapult King Jean-Claude accidentally triggers smashes the siege tower on which Calla and Marie are fighting over Sunni. But since Jean-Claude's catapults are all aimed at the walls of Dunwynn, how come it smashes the siege engines on its own side? Neither catapult nor tower were moved before the misfire occurs.
Continuity mistake: 1-6 A Gummi By Any Other Name: When Cavin climbs the wall of the prison tower, he accidentally dislodges a stone which falls onto the helmet of the ogre guard beneath. But in order to do so, it would have had to fall round a third of the tower's width, since Cavin is climbing out of the guard's sight.