Maybe that amount was entered twice in the register? In that case you would delete the entry (or, to be safe, set it to $0 after writing it down and see if that straightens out the amounts - if it does then delete it).
A glitch here is unlikely. The opening balance is the sum of a ll reconciled transactions in the account. File corruption is possible but likely not the issue - running File->Utilities->Rebuild is something you can try.
