I have not yet dared to try to reproduce it in 1.6.6 because I like my current character and encountering this bug regularly led to me dying – I don't want to die!
I saved while being in a vault.
Then I exited the game.
When I started the game again, I was still in the vault as expected.
But when I took the stairs out of the vault, I did not enter the level I was on before entering the vault, but instead I entered another vault level.
I did not enter this new vault on the stairs but instead somewhere else in the level: Sometimes right next to enemies or even in some of the secret/secluded areas that you have to dig into.
This happened on multiple characters on multiple occasions.
Everytime I fled from a vault just to enter another vault right next to some fresh monsters I was pretty surprised – and dead
