On a test project (brand new empty created just for b17) I’m getting “Undefined tag” errors AFTER I stop playing in the Editor. Once this happens, the tags ARE indeed gone. Opening another scene refreshes the Tag list and all is good until I stop running that scene.
I even tried re-creating the project in 4.6.1p1; which does not display this Tag error when stop playing but after I opened it (and upgraded it to 5) I see the same error.
I would GUESS (rather than assume 
) that creating a simple new Unity 5 project, add a custom tag and maybe the error will display???
If not the asset store package I’m testing is TradeSys and he does have a lite/free version; perhaps that will demonstrate this error.
