Hey this has happened to me before, and to someone else i was trading with.
Its pretty simple concept to understand, when you are trading with someone, if you hearth out or go into arena/battleground at the EXACT moment you click trade - the person who ISNT going into the battleground will get bugged out. They wont be able to hearth or move and it can only be fixed by speaking to a GM - around 1hrs wait sometimes.
Eg.
1. Open trade with someone place in 5 netherweave cloth, dont click 'Trade' yet.
2. Click on your hearthstone and wait for it to start counting down.
3. They will click on 'Trade' and their side of the window will turn green.
4. When your hearth is like .5 of a second away from popping, press your 'Trade' button.
5. LOL @ them
They will be able to chat in guild/area and move around at first, they wont be able to trade with anyone, and if they try to go into a portal they will be completely bugged out. Only a GM can fix this.
It has happened to me before when my mate was giving me an enchant on my bracers, i clicked trade and at the exact moment he went into an arena fight. I was bugged out.
It happened to another mate of mine when i was hearthing, he opened trade and tried to give me a large shard from boss, i clicked on my side of trade at the exact moment i hearthed (accidently as i knew of this problem from my bitter experience) and viola he was stuck in botanica, he couldnt exit the place, and when he tried hearthing the stone went off, the cooldown started and he was still stuck in bot AND unable to even move!
Enjoy. (hopefully they fix this problem as it is VERY frustrating.