I don't know when this happens, but I only notice it when my Wizard wants to cast a spell, but he started the combat with a shield equipped and he has no hands free.
This has happened with my Cleric when I used to have an "empty hand" load-out just to switch to for spells. Sometimes she'd have something equipped that would also prevent casting.
Side Note: Apart from the obvious bug of equipping something that they couldn't normally, I think the whole "load-out" UI could be improved. For example: The "light source" load-out is clunky by forcing the character to always have the weapon from load-out #1 in the main hand. Why not allow the torch to be the main hand, and keep a shield out instead?