Two different occasions I experienced this:
Short term, when fantasy takes over. I could read pages with my eyes but in my head continued the story differently. So I had to read again.... I considered this harmless.
Long duration on two occasions (I was abroad and lots, lots of stress) - where I cannot remember whole hours. When I "woke up" I was afraid I could have done something bad without knowing it... I considered this to be a very, very dangerous sign.
I did not have anything like this for years now - so when getting older, it seems, brain is cooling a little...