Void AP voucher and revoucher without posting between - causes encumbrance issues?

I have been having intermittent problems with AP batches; certain two-way match voucher batches have not been updating the POST code in the F0411 but do update the F0911.  Also, the encumbrance (which should be relieved at voucher match) is not relieving, either.  I could not duplicate the issue with any regularity; this has happened three times since June.

When this happened again yesterday, AP users also informed me that one of them had voided a voucher and re-vouchered it, then had attempted to post both the void and the re-voucher batches at the same time.  They checked the clerk's Work Center, and she had a message indicating "record invalid" and "File cannot be accessed".  Based on this new information, I have concluded that she should have posted the void FIRST, then re-vouchered.