Write a query that guards a report

Write {{dialect}} checks that would catch the data problems most likely to make
this report wrong: {{report}}

Each check returns rows only when something is broken, so an empty result means
healthy. Cover: a sudden count change, nulls appearing in a grouping column, a
duplicate key, and dates in the future.

For each, say what a hit means and whether it invalidates the report or just
narrows it.

What to fill in

{{report}}

What the report claims — e.g. daily paid signups by plan

{{dialect}}

Dialect — e.g. MySQL 8