Catching bad error pointers

Thank you, it has indeed resolved the issue.

I believe the Kernel is also erroneously rejecting all program errors. A program error (PRM5A-496) has %011011 in bits 24-29 and is used by the Wimp to add the Describe/Quit buttons on an error box, I tripped up on this calling the Toolbox so a simplistic case in BASIC is

!&9000=&1B80CB00:REM Toolbox memory error
$&9004="Test"+CHR$0
SYS"OS_GenerateError",&9000

Should be fixed now