Piranha Bytes admitted they made a mistake regarding the number of mages. There's 13 mages in the colony, however Milten joined after the barrier was created, which they didn't think about when creating the intro that explains how 13 magicians created the barrier. The fact is - 12 mages made the barrier, not 13.
The problem with a game like Gothic that has such a hardcore fanbase, is that the fans analyze every rock and spot in the game, so any and all flaws will be discovered. There are a few such flaws in the Gothic games, especially between G1 and G2 (clearly G2 was not planned when they wrote the ending of G1), but all in all I'd say the continuity is very strong compared to most series, considering we've only been able to dig up a handful of mistakes.