I just saw a new user learning kOS stuff do this in JonnyOThan's twitch-plays-ksp stream:

lock heading to steering(90,90).

(A mistake. It should have been lock steering to not lock heading to).

The nasty thing is that this left the core in a broken state requiring reboot - because it LET them clobber the built-in identifier HEADING() with this new user-defined function (the lock), which meant the built-in HEADING() was no longer reachable and could never be used again after that.

Essentially, LOCK shouldn't be allowed to be used with a built-in identifier like HEADING.