I am pretty sure that I had alt tabbed at least once for like 5 minutes before the incident. I wasn't in windows, howewer, as I just entered my inventory and started moving things in/out when shit hit the fan. Yesterday my friend had this same bug by the way - he was prone (I was too when it happened with my MP5!) and entered the settings to change some key bindings and his B95 started clicking its hammer. He had also alt tabbed like 5 minutes ago but didn't do it while entering menu as far as I know.