When wearing the combat gas mask and aiming down certain optics, I've noticed a green object blocking my view, sometimes completely, and sometimes partially.
Namely, this occurs with any optics that involve magnification (e.g. ATOG, PSO-1, PSO-1-1, PU).
I've not tested the hunting scope.
This bug makes the combat gas mask quite unusable with any end game weapons, which problematic, seeing as it is a endgame gear piece.