One Goof, One Plot Hole?
When young David is in Arcana Cabana and Balthazar has already given him the ring, David proceeds to knock the Grimhold out of the hidden spot in the wall. When David picks it up, it magically opens. Why? No thought process or incantation or spell from David, and it just opens up? Why didn't it do that on it's own, before then? Did David, being the Prime Merlinian have something to do with it. If it did, then being the Prime Merlinian would be a bad thing, not good. You'd think that he being the PM, the Grimhold, in his possession, would be even MORE difficult to escape.
Also... there never was a time between the time they met and when they were on the roof after Balthazar saved David from Horvath on the train tracks, that David told Balthazar anything about what he likes to do... yet Balthazar says, "You were the last one Horvath saw with the Grimhold. That puts you on his list. So unless you want him to turn you into a pig, that just lovvves physics..." When did David EVER tell Balthazar he was into physics? They had no contact for 10 years, and he didn't tell him when they met, and he didn't tell him up until this point. So... Like the movie, so I'm not attacking... just wondering.