Occassional errors in VFP 9

Hi experts,

I would like just your opinion to this subject... How often do you observe some occassional error in VFP 9? The last example at my side is:

		SELECT tmp58orders
		IF TYPE('stornomark')='U'
			ALTER table tmp58orders ADD stornomark c(1)
		ENDIF 
		IF TYPE('typemark')='U'
			ALTER table tmp58orders ADD typemark c(1)
		ENDIF 
		REPLACE stornomark WITH 'S' FOR s__storno
		REPLACE typemark WITH 'M' FOR s__type<2
		REPLACE typemark WITH 'S' FOR s__type=2

tmp58orders is a writable cursor created by SQL SELECT (one output row only) and the error reported was:
1) Variable stornomark does not exist   ...  in the first REPLACE command
2) Variable s__type does not exist  ... in the second and third REPLACE commands
... more missing columns...

This code works in 99.9% of cases but it seems the cursor has totally different structure than I could suppose this time... and the first ALTER TABLE command failed without any error report whereas the second one succeeded...

I have just one explanation: "the fox is drank"...

It is also possible the first ALTER TABLE command crashed resulting in some unexpected data structure, but who knows...

So the only remaining question: How othen does this happen to you?