Usability guru Jakob Neilsen tells us about small annoyances in user interfaces that compound and cost you money.
Neilsen highlights the use of drop-down menus to select state abbreviations in e-commerce sites. This example resonated with me, because I have definitely been annoyed by such menus. Neilsen notes that Amazon offers users the ability to simply type the two-letter state abbreviations. Either way, Neilsen recommends web sites "validate that the ZIP code/postal code corresponds to the state, province, or other locality entered by the user".
The larger point is that your product manager should be framing the metrics to measure these annoyances, QA should be testing them, and your web designer should be minimizing them.