>Some more info .. The Report Generator appears to be the culprit .. I can replicate this every time in the command window .. you can use Wordnum() instead of Words() .. you get the same result ..
>
>*Create a test variable ..
>t=repl("X,",100)
>* set the library
>set libr to foxtools.fll addi
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>
>* Create a table and then a report for it
>Create test && just a couple of text fields and add some dummy data
>create report test && use quick report .. nothing fancy .. save it (DON'T PREVIEW)
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>
>* Now either run the report (Preview or Print) or modify it and use the Preview option in the report builder
>Modify report test && this time PREVIEW it
>* or ..
>report form test preview
>
>* Now run the test again ..
>?words(m.t,",")&& this causes the error "API call caused an exception"
>* now try again
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>?words(m.t,",") && this works fine
>
>
>Either running or previewing the report (any report) will cause this error the next time Words() (or Wordnum() for that matter) is called. The same holds true for a printed report ..
>
>This even works this way ..
>Start VFP6
>Modify a report and PREVIEW
>Now run the test ..
>
>t=repl("X,",100)
>* set the library
>set libr to foxtools.fll addi
>?words(m.t,",")&& this causes the error "API call caused an exception"
>
>The Report Generator not only screws up an existing library it will mess up future ones ..
>
>This is , to say the least, extremely frustrating .. one wonders what else the Report Generator is screwing up ..
>
>Note: My startup VFP code merely assigns some hotkeys and sets some colours ..
>
>Thoughts ????

I've tried it several times. I did get a 'VFP caused an exception...' when I tried to select fields for the quick report, but not in the manner as you describe.

The only thing I wonder about now is the OS. So...What OS are you running? Regardless, since you seem to be be able to replicate this, I'd suggest you submit your findings as a bug report.