I’m sorry, but the most classic, rookie mistake you can make in SQL is to turn a table in on itself and create a Cartesian Product.
Which I did.
On our shiny new server.
It ran for six hours before I caught it.
And somehow the delivering of the massively huge mapping project in less time than it has ever taken, ever, takes a huge back seat to the classic rookie mistake of someone who doesn’t even manage classic rookies.