A vendor handed me a dataset with a confident headline number attached. It would have been easy to take it at face value and build on top of it. I sampled it against the real source instead, and the headline was flat wrong.
This happens far more than you would expect. Purchased lists, enrichment vendors, even internal marketing numbers all tend to arrive polished and unchecked. The polish is not evidence. It is just formatting.
The cheap habit that keeps saving me: before anyone sends, dials, or reports against third party data, pull a small random sample and check it by hand against the source of truth. Ten or twenty rows is enough to smell a problem.
A five minute spot check up front beats discovering three weeks later that a whole campaign or analysis was built on a claim that was never true.