The composite key I got the wrong way round, again

Equality before range, for the fourth time in ten years, on an index created in a hurry.

-- created
KEY idx (created_at, status)

-- the query
WHERE status = 'pending' AND created_at > ?

-- rows examined: 412,000

-- corrected
KEY idx (status, created_at)
-- rows examined: 1,204

This is the most common index mistake and knowing it does not prevent it, because the column order feels like it should follow the query text rather than the predicate types. The check that catches it is reading rows in the plan rather than checking that an index is used — an index being used is not evidence that the right one exists.