Alerting after the fact
is not data quality
There is a meaningful difference between a system that notices bad data and a system that refuses to serve it, and most teams have the first one while believing they have the second.
The failure mode nobody plans for
A source system changes a field. The load succeeds, because nothing about it was technically invalid. The numbers move. A dashboard shows the new figure, somebody presents it, and the error is discovered two days later by a person who happened to know what the number should have been.
The damage is not the wrong figure. It is that everyone who saw it now discounts the next one, and confidence takes far longer to rebuild than the pipeline took to fix.
What a gate does differently
A gate runs before the data is published, not after. It checks the things you already know to be true: this column is never null, this identifier is unique, this total should be within a plausible range of yesterday, this reference should exist in that table.
When a check fails, the load stops and the previous good version stays in place. Reports keep showing yesterday's numbers, which are correct, rather than today's, which are not. Somebody is told immediately, and the people in the meeting are never exposed to the bad figure at all.
Serving stale data on purpose
This is the part that feels wrong the first time. Holding yesterday's data looks like a worse outcome than showing today's.
It is not, and the reason is asymmetry. Slightly old data that is correct leads to a slightly delayed decision. Fresh data that is wrong leads to a confident decision in the wrong direction, and to everyone downstream trusting the platform less afterwards. One of those recovers on its own.
Where the checks come from
The useful ones are rarely invented in a workshop. They accumulate from incidents. Every time a number turns out to be wrong, the check that would have caught it becomes a permanent rule, and the same failure cannot happen twice.
That makes quality a ratchet rather than a project. It also means the first few months are the worst it will ever be.