Game works in editor; not builds; "GameObject has undefined tag"

I have been working on a chess game on Unity 5. It works correctly when testing, but after I build to web or exe, and run the game, every time I click a piece, the console spits out "UnityException: GameObject has undefined tag!" But I checked, all the pieces are tagged, and this is not an error that ever occurs when playing in development mode? Anyone have any ideas on how to narrow down what's causing this/how to fix? I'm very new to unity (this is only my second project) and would love to get this working.