I think that it is partly because the drugs generally have an amnesiac effect at lower doses, so if you don't give enough of the drug to cause amnesia, there wi

I think that it is partly because the drugs generally have an amnesiac effect at lower doses, so if you don't give enough of the drug to cause amnesia, there will almost certainly be pain, too. But maybe there's a more cynical interpretation that it it "didn't happen" in a liability sense. If the patient has no memory of the pain then they have no reason to sue you for messing up the dosage. Incidentally I recently watched through Severance and I can't help but see the parallels to the show.