I agree that you should correct a deficiency immediately, but I think it should be done with utmost clarity.'Casually' implies that you as a manager might not care about the outcome or might not express your true concern about the incident. I always make it very clear that an expectation has not been met.

Several years back I took a "Crucial Accountability" course on having accountability conversations. In their process there were four phases to every accountability conversation: 1) Reiterate the expectation 2) Describe the incident in question 3) Ask the employee to describe the gap between the expectations and their behavior in the incident 4) Have a conversation about what we're going to do about it.

I've used this model many times and it is very effective. Usually the employee leaves the conversation understanding what I expect and assures me that they will try to do better. Sometimes it requires follow-up correction, or further conversations, and sometimes it requires an action-plan that we work on together. Rarely does it lead to an untenable disagreement in how things should be done (and you can guess where that leads). Also rarely, the expectations will be amended or more clearly expressed to everyone.

I do think getting their perspective is necessary to understand if the expectations were incorrect, or if my understanding of the incident was incorrect. But it is essential that the conversation always start with reiterating the expectations.