Special tip, especially for all those "hyper-key" folks out there, but good for everyone to know…
I tried to assign command+shift+option+control+period as "Cancel All Macros" for Keyboard Maestro, and couldn't get it to work.
I didn't get an error or a system beep, it just appeared as if nothing had happened. But something was happening in the background, it just took several minutes before it became apparent.
Turns out that command+shift+option+control+period is a macOS reserved shortcut which will produce a "sysdiagnose" file in /private/var/tmp/ (mine was 350MB, compressed).
As far as I can tell there is no way to override or re-define that keyboard shortcut.
Which shortcuts? This sysdiagnose is a low level feature, it is not meant to ever be invoked except in weird debugging situations. Unfortunately the concept of a Hyper key made this a poor choice of keystroke, but it is understandable in the absence of the Hyper key to make it as complex as possible.
There are a few other hard-wired shortcuts, like Command-Tab (which Keyboard Maestro goes way out of its way to work around). But I don't see any that can't be changed in Mojave that used to be changeable…?
Well, actually I was thinking of a statement by you a few months ago, to the effect that some "system shortcuts" could not be over-ridded. Perhaps my memory is faulty.
Yes, in Mojave you cannot trigger a hot key using a simulated key (eg Type a Keystroke) if the key is an arrow or a function key (although weirdly it works for regular keys).