If you try to use the optics on a rifle that has been lowered(by double clicking control) at the same time when moving forward (keeping W pressed)

the character will then continue to move forward even if you release 'W'.
the following keys become unresponsive W,A,S,D,Z,X,C

The only way i've found to stop this is by pressing 'V' (step over, vault animation) or by switching to pistol via 'spacebar' or 'middle mouse button'