This usually happens after me and my friend meet up from far distances where one or both of us have invisible weapons. Although you can fix it by placing the weapon on the ground and picking it back up again. Still quite annoying.

I usually get this happening after my game crashes and I log back into the same server, although I am able to stop the attacking by left clicking once. There have also been random instances where I had my gun in my hand and accidentally left clicking once caused my character to continue shooting the weapon until I left clicked again (possibly due to bad server performance).