When you move the mouse into the "hot corner" to activate Screen Lock, there is a brief interval (maybe only a second) during which if you move the mouse out of the hot corner, the screen is unlocked!!!
This is idiotic. As I have said many times, Apple should abolish the use of "hot corners" to enable screen lock, and use their own standard GUI guidelines of providing a single PUSH BUTTON to be clicked on in the mouse in order to activate screen lock.
This could go in the Apple menu, along with Sleep/Restart/Shutdown. Or it could be a Preference to "Show in Menubar" (like for Clock and Displays). Or it could even be a program you start from the Dock (but supported, not the old kludge of drag the Screen lock internal DLL into the dock).
But it must be clear, unambiguous, and irrevocable; once it's pressed you should not be able to do ANYTHING until you have entered the password.