Pretty consistently, when the anti-grav buff wears off during a sex animation that doesn't re-apply it (tentacle grab, butt balls) the character will stay in anti-grav state permanently. Depending on where this happens, this can easily throw you off the map or softlock you on the ceiling, and since the debuff timer has already expired, you can't just wait to get out, you have to restart the game to return to your last save.