In a compiled exe, out in the real world, being used by real users. This error randomly appears!

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Visual FoxPro Reference

END TRANSACTION command cannot be issued without a corresponding BEGIN TRANSACTION command (Error 1591)
Contain transaction code between matched BEGIN TRANSACTION and END TRANSACTION pairs.

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The offending code from my program is below:

SELECT INVOICE
BEGIN TRANSACTION
DO CASE
CASE lnanswer = 6
REPLACE INVOICE.status WITH "P"
REPLACE INVOICE.oktoprn WITH .F.
CASE lnanswer = 7
REPLACE INVOICE.status WITH "?"
REPLACE INVOICE.oktoprn WITH .F.
ENDCASE
Thisform.pageframe1.page3.stat1.refresh
lmsuccess = TABLEUPDATE(0,.F.,'INVOICE')
IF lmsuccess
END TRANSACTION
ELSE
=MESSAGEBOX("Invoice Status failed to update successfully",16,"Transaction Error")
ROLLBACK
ENDIF

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Any suggestions would be gratefully appreciated.
Thanks
S Ramsey