I was on my level 45 Arcanist, unknowingly using a Arcanist/Summoner only weapon. I tried equipping my Scholar soul crystal multiple times and I kept getting an error saying

"You may not switch to Scholar from Arcanist. You must first change your class to Arcanist."

Not only was I already on Arcanist, but the first sentence of that error message is completely incorrect. It was very confusing and it took me awhile to figure out why I couldn't switch to my Scholar job. It was because my weapon equipped was for Arcanist/Summoner only, Scholar could not wear it.