I can now reproduce the problem.
Update: what happened was, as mentioned above, I added a secondary system command and that's when it freaked out.
I added the command - nested within the first System command. Like this:
System>on start of layout>player>set angle....
System>every tick>player>set angle....
Whereas it should be:
System>on start of layout>player>set angle....
new command>blah blah
new command>blah blah
System>System>every tick>player>set angle....
I think I got Events (which can't be nested) mixed up with Actions (which can).
So, once again, thanks be unto me for figuring it out, and thanks to you for pointing me in the right direction.