Re: Security 101:
Created an odd situation yesterday.
iMac. System was working fine beforehand.
Problem: Unable to log into an iMac. I had applied/installed security update 2016-001 (10.11.6) manually via combo update, rebooted. Had been using a physical (non apple, 2 button) mouse and keyboard before this update, worked fine.
After install, at login screen, unable to click (left) mouse click to return focus to login.
Powered off, restarted, this time focus was showing flashing caret correctly, managed to enter password using a physically connected keyboard.
On reaching desktop, had mouse movement, but no (left) click, with a physcially connected mouse. Mouse appearing to be drawing selection boxes (signs of a held down left mouse button)
(Tested under Windows: The iMac dual boots Windows 10 1607, the physical mouse/keyboard was working correctly under Windows 10 1607)
Back in macOS: Had to use keyboard to get to System Preferences, (eventually) searched bluetooth.
It turned out, the iMac had enabled Bluetooth during the update, a Bluetooth Magic Mouse (which was switched on, on a nearby desk but stuck under paperwork, holding down the mouse click), had connected automatically to the machine.
This prevented any physical mouse from operating on the machine itself (as the magic mouse, was generating a held down left click response)
Disabled Bluetooth, rebooted, got back control.
So a Bluetooth Magic Mouse can cause havoc, if its ready to connect, during the installation of combo update Security Update 001 (10.11.6). Certainly odd, though, luckily I guessed it might be to do with bluetooth and a nearby device.
i.e. A malicious sellotaped down (or in this case a paperwork held down) click button, bluetooth magic mouse seems to have the ability to cause havoc.
Still trying to piece together how it all came about though.