Nobody sets out to build a confusing product. These mistakes creep in gradually, through good intentions and shipped-in-a-hurry decisions, and they're hardest to see from inside the team that built the thing.
When every button on a screen is styled with equal visual weight, the user has to work out what actually matters. A single, clear primary action per screen isn't a stylistic preference, it's the difference between a decision and a guess.
Most UX gets tested on the ideal scenario: fast internet, correct input, no errors. Real users mistype, lose signal, and hit edge cases constantly. A product that only feels good when everything goes right will feel broken to a meaningful share of real users.
Surprise shipping fees, sudden subscription terms, or a price that only appears at checkout are one of the most reliable ways to lose someone who was already convinced. Show the real cost early. Trust, once broken at the finish line, rarely gets a second attempt.
A beautiful interface that takes eight seconds to become interactive is not a beautiful interface to the person waiting for it. Test on a throttled connection, not just the office Wi-Fi, before assuming the experience is fine.
Teams that only test internally develop blind spots fast, because everyone already knows how the product is supposed to work. Watching one real, unscripted user session tends to surface more problems than a week of internal debate.