>? 'PRESS ESC TO STOP'
>* NEXT GO INTO A INFINITE LOOP
>FOR I=-1 TO 1 STEP 0
>NEXT
>? 'END'
>

>
>Then, it check NULL, but not check 0.

I can kind of see this one: it's similar to

DO WHILE .T.
ENDDO

with an EXIT in the middle to break out when the proper conditions are met, so I don't see the need to error here.