Legend.
My bugbear is that the self-service checkout won't accept my sell-your-shopping-data-for-a-discount card until I've punched the "Pay now" button -- at which point, without the slightest hesitation, it says "Don't forget to scan your club card!" And I'm like "I haven't forgotten, you just wouldn't accept it until Just F#$&*#$ Now!"
I acknowledge this is not entirely rational.
(Oddly, the staffed checkouts accept your club card at any point, not just the at end.)