I would consider myself a bit of an expert in mythology and the paranormal.
My thoughts on this are:
-Firstly, it depends on the mirror.
-Second, if there are two mirrors then you can, in theory, create a gap in the physical plane to the astral.
-There is also a psychic ability known as Phasing involving the molecules of the body being made intangible for a short time.
-With Phasing, you could theoretically enter the gap into the astral.
-Some abilities are unconsciously manifested in times of stress and worry.
I believe this was what accidentally happened. If you want more information on the astral/etheric planes please do not hesitate to reply to this post.