These aren't beginner errors. They're the specific things that slip past skilled, careful developers precisely because they're easy to overlook when you're close to the work. If you've shipped sites, you've probably let at least one of these through.
1. Forms that submit but don't deliver
The form says "thank you," but the message never arrives — a broken integration or a spam filter eating it silently.
2. Mobile layout breaks on real devices
It looks perfect in the browser's device emulator and falls apart on an actual older phone.
3. Broken links after a content shuffle
Pages moved during the build, and internal links quietly point to nowhere.
4. Slow load from unoptimized images
A gorgeous hero image that's a 4MB file dragging the whole page down.
5. Checkout edge cases
The happy path works; a discount code, an out-of-stock item, or an international address breaks it.
6. Missing or duplicate page titles and meta
Easy to skip, quietly bad for SEO.
7. Old URLs not redirected
The redesign went live, the old addresses 404, and the client's search ranking tanks.
8. Accessibility gaps
Missing image descriptions, poor contrast, keyboard traps — invisible unless you check for them.
The common thread: every one of these is invisible from the builder's chair, because you test the way you built it. That's not a skill problem; it's a proximity problem. An independent QA pass catches exactly this class of bug before your client does. It's why even great developers benefit from a second, neutral set of eyes.