After the end of a magichange, you keep the damage formula of the magichange weapon instead of getting back your initial weapon damage formula.

Simple way to reproduce the bug :
- Stage 1-1 to make it safe
- Lv 1 character with a strong sword
- Magichange with a prinny to get a gun
- Wait for magichange end (or magichange 2 to make it faster)
- Attack an enemy. Enjoy your 7 damage with a 1800 ATK sword

Unequip and reequip weapon 'solve' the problem.

You can do the same with any weapon (ie gun then magichange sword), as long as the weapons use different damage formula.