What do you call a reassurance that was given after a person has 'gone too far'?

When you actually encounter it, you could certainly call it a belated warning.

belated - happening or coming very late or too late

Since the warning was always underneath the container, it doesn't really make sense to call it "belated" (or anything else referring "lack of timeliness") before it's read. The best you can do there is say it's badly positioned, or some other reference to position, rather than [future] time of reading.

Urban dictionary has this word: afterwarning.

A warning of a past event.

Not bad for a neologism.

An "untimely" warning is one that comes at an inappropriate time, which could mean "far too late to do any good" in the proper context. It's not exactly what you are looking for, but it can be used to mean that given the rest of the context.

Perhaps dilatory warning

characterized by procrastination; tardy

Or after the fact advisory

Too late; after something is finished or final