I tagged to just test two horses for my 49 lvl fish alt with full inventory. (test return speed and figure which horse to keep on fishing alt)
Well looks like I killed some critter on way and now I AM FORCED TO GET XP ON ALT. 
So Im forced to create alt because ganking fishers is allowed and now I'm forced to create new 49 alt because some invisible mob was on my path. 

Oh well more money for other games.

You can send a ticket and GMs can dissolve the tag without gaining the Exp. Its a bit annoying, but it solves the issue somewhat.

Now I'm curious, shouldn't that quest you need to complete to get to level 50 prevent you from getting XP anyway? My level 49 character gets XP all the time when killing mobs e.g. for gathering, but she still stays level 49 because she hasn't completed the quest for level 50. Or is this  ignored when tagging a character? If that's the case, that's clearly a bug and should be fixed.

You main char gets exp and your tag has to claim xp before you can untag. If you're 49, you have to do the quest in order to claim the exp so that you can untag, otherwise you can't untag at all.

Ah, thanks for the explanation. So you can't just claim the XP without doing the quest and it goes to limbo like any other XP normally would in this situation? This looks like a serious bug to me, that really should be addressed. 

Ill use this opportunity to complain abotu the tagging system too because like sometimes you will have done exactly nothing since you tagged and it will STILL not allow you to untage saying SOMEHOW the other char gained exp. Im serious, recently i just moved my character a 100 meters, i swear i cant see what i would have killed, im in a town, i didnt process anything, nah still had to log on the alt. This is so badly designed. First of all... why cant my tagged char claim the exp even if im not logged on that char? that should be possible so i dont have to waste time switching to the other char.