I have a multiple records data block. The user can update 2 fields on this block . A use modifies a field and doesn't save . In another session , another user tries to update a record , the message 'unable to reserve record' appears ..
In that case some one will have to interfere and kill the session of the user locking the row.
One more thing. If there was a way to prevent this that would lead to inconsistent database data. On the other hand you can set timeout limits or user inactivity timeout on client/middleware to logoff users not saving changes for a long time.
i do recommend u using a key-exit trigger or when window close trigger
Form level or similar trigger to remind user to save the changes made before closing the form & u can follow my guide lines in the following thread to follow up this error Error - could not reserve record