Sequelitis: The fourth book can be considered to suffer from this trope. It feels very disjointed from the rest of the series, has a completely different style of writing and its full of retcons. To list a few of its flaws:
Joram somehow made another Darksword, after he destroyed the first one in the third book, while the first book established he required the help from a catalyst during the forging process. This book retconned it in that a catalyst only makes a Darksword stronger, but is not required.
Mosiah becoming a member of the Duuk-Tsarith, the law-enforcers and warmages of Thimhallan. In the first book, it was told that the Duuk-Tsarith are chosen from childbirth and trained at a very young age. Mosiah grew up as a farmer, not a warmage.