Finding a stray transaction

[Grant]

I'm trying to find the transaction or transactions which are
preventing one of my gnucash accounts from reconciling.  The latest
month I'm trying to reconcile checks out itself, so there must be a
problem in a previous month.  Does anyone have any advice for tracking
down the problem?

The only thing I can think of is manually un-reconciling each
transaction in each previous month so I can test re-reconciling each
month until I find the problem.  That would be an insanely laborious
process.

- Grant