PRINT AT 23,0 and FOR NEXT Loop not working after Fastcall

[boriel]

But I can implement a way to optionally resignal "OK" if you want.

I not fully understand what you mean with this. If you like, you can implement it. But as I now know the circumstances about it, I can also avoid "Printing out of screen"

you can also PEEK 23610 (ERR_NR). ZX Basic stores the error there. It should be the same number you get when you exit to basic minus 1, so for "0 OK 0:1" the code is 0, so poke it with 255 (which is -1 in 2 complement). That is POKE 23610, 255 before exiting to Sinclair BASIC will clear any error silently.