A friend brought me a M4 with M68 CompM2 optics attached. He carried it in his hands while having his own gun on back.
He dropped the gun, I picked it up. Then I had this weird bug: when aiming I looked into the weapon instead of thru the optics (see screenshot).
Removed the optics, aiming down the sights works fine. Re-attached the optics, same bug.
Re-logged on the same server, bug is gone, optics work fine now. {F30224}