When NOT to use your product

In Part 2 of Lulu Brown's The gentle art of product-rep self-defense she mentions that:

Some of the best reps I've ever worked with have flat-out told me when their product was not the right one for the job. It made me want to use them again on another project as soon as I could.

This is a common result, and an important one to keep in mind.

Research has documented this effect at auto mechanics that use the same technique: they examine your car, tell you everything that's wrong with it, and tell you which repairs can wait or are unnecessary. The result is you feel more comfortable paying for the rest of the repairs, because obviously they're not trying to rip you off.

When the mechanic or dentist does this, it sometimes has the appearance of being a game. After all, they took a $500 job off the table and sold you $2,000. When a product rep does it, it tends to be more all-or-nothing. Still, despite the seemingly higher stakes, it is a good practice. It is as important to know when not to use your product as when to, because using your product inappropriately, or not to its best ability, will create a negative experience for your client.

What sales have you passed on? Or, as an architect, have you had a rep tell you not to use their product? In either case, what was the longterm outcome for the relationship?