With the following admission I'll definitely be outcast from SAP Basis society - but they forced me (yes they have ways to do that here), to perform the upgrade of DEV and PRD in parallel to "save time", to catch up on deadlines that were in danger of being missed due to a problems experienced during the Sandbox upgrade.
With fewer resources available, the DEV system is now ~2 days behind schedule of PRD, but the Devs went on the (not-upgraded) QUA system and used that as the new source system.
I know this is a messed up procedure, but after two successful test runs on Sandbox systems, the(ir) reasoning was, "what could possibly go wrong during a third run?"
Please folks don't crucify me, I know we should still have done DEV first, but everybody here got fixated on getting PRD done asap and on time.