The Problem (tl;dr section)While leaving a dungeon in Daemonhiem, there's a small chance you are switched from Lunar/Ancients to Modern Magic. If you cast Spellbook Swap after leaving Daemonhiem, you also may remain on Modern Magic after you cast your 1 Modern Magic spell.
While leaving a dungeon in Daemonhiem, if you have a standing Vengeance status, it will carry over to the Overworld, granting you the power of Vengeance even though you casted it in the dungeon.
I just wanted to warn you all about this bug I have encountered quite a few times. I also wanted to see if anyone else had encounted the same problem, to make sure Jagex isn't out to get me. Anyway, thanks for your time, and if you want to speculate about the possibilities of abuse, that's fine with me.