AP Payment issued for invoice in Needs Revalidation status?

A user modified the accounting distributions on an AP invoice that was in "selected for payment" status; and after the check was issued, validating the invoice has assigned a Line Variance hold that will be unable to be released because we're unable to discard the additional GL coding.

First, how did this happen, any ideas? I thought once an invoice was selected for payment, nothing more could be done to the invoice; or that if a change had been made, the invoice would be unable to pull for payment?

Second, how do I keep this from happening again? I'm not sure we would have noticed it until the check was out the door, cashed, etc.