They just teach them how to do it right.
They teach them nowadays, before it was basically DO WHATEVER YOU WANT COS' ELDAR LIFE IS FREE.
From birth.
Less time to fuck up if there are only like five seconds after your birth when you can fuck up before brainwashing.
Constantly.
Good for them, that they need some constant reminders.
For hundreds of years.
Eh, they might not be that great again if it takes them hundreds of years to grasp such basic concept as "NOT FUCKING AN CHAOS GOD INTO EXISTENCE".
And have mary sue excellent memories.
That explains why they do that, though it would be hard to forget FUCKING AN CHAOS GOD INTO EXISTENCE, AND BASICALLY PSYCHIC GENOCIDE OF THEIR OWN RACE.
And are all natural psykers so it's a lot easier for the knowledge to stay ingrained within the culture.
The all-natural psykers was the reason they fucked up in first place.
And also they don't actually do cantations all that much.
Not anymore, not really.
I'm talking about the method one prays to the Emperor, here. Not psyker shit. And to some extent the Cult Mechanicus, but ehhh. They don't kill people so much as issue plasma guns.
The thing is that Emperor might actually answer those prayers (Sisters of Battle cause miracles on daily basis), and so might Chaos Gods, so "wrong cantations and rituals" might mean "OPENING A FUCKING GATE TO WARP IN MIDDLE OF TERRA" or actually mentioned "FUCKING AN CHAOS GOD INTO EXISTENCE", you don't need to be psyker for psychic entities to act... and Cult Mechanicus cantations are basically maintenance protocols so if you screw up you might end up exploding the entire planet with acient toaster... or, if the thing they're talking to is advanced enough to be called an "Machine Spirit", screwing up might cause it to go on murderous rampage.