Machine loses pos?

So I am curious to what would cause a machine to just lose itself and go wherever it feels. Machine in question is a mazak fh horizontal. Just tonight I had the machine soft limit on a tool path. This program has been run for years 1000's of times. On another occasion I have seen it basically shift it's work offset and cut a part in half because it ran the code fine, just about 8" off! It is not operator error, what else could cause this?

I rebooted the machine, started the tool and it is running fine. Last I saw it do this was maybe a few months ago...