Retargeting

Showing ads to people who already visited, usually the cheapest and most over-credited spend in an account.

Retargeting looks brilliant in platform reports because it reaches people who were going to come back anyway. The honest test is an incrementality holdout, not the reported ROAS.

Cap frequency and exclude recent purchasers — most of the waste in a retargeting budget is buying the same person twice.

It is the line most likely to be over-funded on the strength of numbers nobody has stress-tested.