I searched and found nothing similar to this so I figured it's worth sharing. If your guild has the guild achievement "Have group will travel" apparently there is a bug that will summon someone from a completely different realm to your realm.
I don't have a screenshot and can't test it again because I just got kicked off by the battlenet servers going down (+ cd on HGWT) but it seems pretty straightforward.
Here's what happened:
- Queued for dungeon with a friend from my server.
- Left dungeon at the end, there was one warrior still in party from another realm.
- Used Have Group Will Travel to summon my friend to me in Ironforge.
- Warrior from other server appears in our Ironforge as well.
I couldn't trade with him unfortunately, however he was running around and definitely appeared to be on our server somehow. This may have been a glitch but I'm thinking it is just an oversight by Blizzard with regards to how this spell works. If someone else can test to confirm that'd be appreciated.