I have seen a few similar problems on SE. None of them match exactly, and unfortunately none of their solutions worked either.
The problem: Alt + Left key locks the screen. This is also the Chrome browser shortcut for back, so pretty frustrating.
Solutions tried: I've looked through all the keyboard shortcuts and nothing matches this particular combination. I can't see anything in Tweak Tools either.
When: most frustratingly, intermittently: when I first launch Chrome it doesn't happen. When I first start up my machine it doesn't happen. Something triggers this behaviour in my normal workflow, and then for the life of me I have no idea how to turn it off without restarting my system.
Set up: Ubuntu GNOME 16.04 on an Dell XPS 13