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).

Same issue with me. but it only happens once in awhile though when i'm trying to melee a zombie.

Same issue: usually happens just when logging in to a server while holding a weapon in hand, it keeps raising the gun (luckily not shooting) indefinitely, no matter how much you press space. To me, the only way to get rid of it (without shooting the gun) is to take a melee weapon in hand, which causes the character to start swinging it, and then press left click once to stop the action. Then switch back to the gun.

This is caused by shift tabbing at the same time as pressing any mouse command. (If it is the version where you left/right click and it stops zooming/shooting) It automatically holds the bind down. When you shift tab dont click on chat to fast.