I had the same Bug/ System hiccup just this week... scared the hell out of me...
I changed my password and my ava (ava is a new one cause I could not find the old one). With help from shin I even found the other player who was logged into my account. I send her a IGM and told her in a nice way that I know she was in my account and that if this should ever happen again, she should not change the ava, just log out and leave it be... she was very friendly and told me she did not touch anything else, when she realized t wasn´t her own account.
Unfortunatly, we can´t do anything about it. Just this:
If you ever get logged into a account, thats not yours,
- leave it as it is,
- don´t do anything with it,
- just log out of it.