The page loads and it is empty
Somebody opens a page and there is nothing on it. No error message, no spinner stuck forever, no red in the console — an empty list where there used to be rows. Refreshing does not help, and it works fine on your account.
Why it is hard to see
Every instrument reads healthy, because every instrument is measuring the wrong thing. The request returned 200. Nothing threw, so an error tracker has no event. It was fast, so a performance tool has nothing to flag. The query ran and succeeded — it simply came back with no rows.
The usual cause is a permission rule that is missing, newly added, or subtly wrong: a policy that filters everything rather than something. A form that submits and does nothing is the same failure wearing a different hat, and so is a 401 the interface never surfaces.
What Sixty does about it
The number of rows an operation returns is measured and compared against the previous release. A query that returned twelve rows and now returns none is a finding, in the same way that one returning thirty thousand is — the shape of the answer changed, and nothing else on your stack is watching that.
Requests turned away with a 401 or a 403 are counted the same way, so permissions that tightened yesterday show up as a rate that moved rather than as silence.
What it does not do
It can tell you what broke and how often. It cannot tell you who hit it — no session IDs, no user IDs, no recorded sessions — so reproducing it against a specific account is still your job.