And as all that matters is what happens in the fiction, if drinking this coffee is important enough to be relevant then it should have been roleplayed through while the characters were still having breakfast....
... because of this. Retconning as described here would, I think, qualify as cheating pretty much everywhere.
So, the dividing line pretty much becomes one of whether something like this can or does have any mechanical impact. Unless the coffee was poisoned or gave some mechanical benefit or whatever, whether or not you drank some this morning has no real bearing on anything other than flavour. Flavour is good. Go for it!
Contrast this with retroactively introducing a hidden gun in the kettle: irrelevant if no-one ever finds it but which does have mechanical impact the moment it's discovered by anyone. This is bad.
Or the Potion of Longevity, introduced retroactively with the specific intent of giving a mechanical advantage now. This is worse.