Pete,
Probably a corrupt object code field. When you remove the code and then put it in again VFP has to recompile it for the Objectcode field in teh scx.
Make a copy of the scx/sct. Then
USE The.scx excl
REPLACE ALL ObjCode WITH ""
PACK
USE
COMPILE FORM The.scx
and see if that fixes things.
JimB