Discovered today, and just reported through the Help > Report a Problem system, shortly after noon PST.
Quicken Premier Windows, Version R64.35, build 27.1.64.35, current subscription, Windows 10 Enterprise
Given a transfer transaction, such as payment from a checking account to a credit card account, If one side of that transaction is reconciled, and you try to change the value of the payment, you get a pop-up warning asking if you really want to change the reconciled record. Good.
But if you change the value of the other side of that transaction, which is not yet reconciled, Quicken very obligingly changes the value of the not-reconciled transaction, AND changes the value of the corresponding reconciled transaction without warning.