Re: ERP == Enterprise Reduction Programme
A couple of things you forgot to add to your list: "sacrifice a virgin at midnight on the day before switch-over" and "hope like hell".
There's always that. But nothing beats a dress rehearsal run a few days before the actual go-live.
I was involved in a client rescue a few years back. They had implemented AX2012 with a vast swath of customizations that sorta-kinda worked ok, but they never wanted to test the data migration code. They assured us that their code was perfect. So come the go-live, and their system starts spitting out JSON files the size you've never seen before. The AX code groaned under the load, but dutifully processed the data. As you might have guessed, their data was wrong, and every single journal and customer state they shipped in had issues.
Every. Single. One.
So rather than abort the go-live, they decided to press on, and thus began the cleanup work. A year later, the original team started to break up for varying reasons, and I was brought in to keep going with the cleanup. That's the we discovered that not only was there bad data screwing up current transactions, there was bad code writing vast amounts of junk to the database causing further problems.
The remediation work took another six months, until the new CFO of the company in question said to his finance team to just journal out whatever the issues were and move forwards. We got the system mostly debugged, but by then the company, which had been acquired by a much larger company, decided to break away and found a new partner to continue with.
Our PM didn't stand up to the customer PM enough to demand go-live testing. And this was the result. For want on 30 minutes of go-live testing, the entire project and customer was lost.