How does this happen?

I believe it happens when you heal your hp at the same time as it drops to 0. Atleast something like that.

Millumi wrote:

How does this happen?

I believe this has been reported a few weeks back. This kind of bugged just appeared to all of us the time after the Sakura Island etc. patch was applied. I suggest you should also submit an RT relating to the problem as a follow up on this matter.