In BB5, I’ve noticed that if I save during a lair mission, a turn or two later the game will freeze at the start of the alien turn, as if it’s ‘thinking’ about the turn, but nothing happens. No input is accepted, and I have to task switch out and forcibly quit the game.
Upon reloading the save, the same thing will happen, meaning I have to backtrack to the last geoscape save.
I don’t normally save during regular tactical missions, so this is where I have noticed it, but I have a faint recollection that it might have happened in other missions too, before I noticed the pattern.