>>That's the same version I'm running. I wonder if it was different in VFP6, I don't have it to test.
>
>It does not work in VFP6 SP5 either. I can't test any earlier than that. Looking at the help for error 1851 (Selects are not UNION compatible), it says "Data types or sizes of fields being projected in each SELECT statement are not identical" (emphasis mine).

Which does not explain why the following does not generate an error:

select lastname from customer union select space(1) from inventory

They should both either work or fail. I did post this as a bug with all due credit to Roi for bringing this up.