It makes no sense to enter a reconciling date before the last successful one, unless one intends to unreconcile later transactions. When trying to reconcile to an earlier date, a warning could explain why this is a bad idea.

Misleading scenario that a warning could prevent:

After reconciling transactions from march to december, I need to add data for february, before all reconciled transactions. I take my bank account, intending to reconcile at the end of february, because I think the reconciled amount would be the sum of reconciled transactions before the entered date. This is wrong, it is the sum of all reconciled amounts independently of the reconciling date, so the numbers will be wrong in my point of view. I should enter my data, and reconcile always the same amount at the *last* reconciled data.