The expensive misses are boring
Sites rarely launch broken in visible ways — someone would notice. They launch with a no-index left on, a form that goes nowhere, or redirects that were never written, and those cost weeks before anyone spots them.
1–3. The ones that hide your site
No-index removed from production, robots not blocking, sitemap generated and reachable. Any one of these can keep a site out of search entirely, and all three are one-line settings — the same trap as losing traffic in a redesign.
The three worst launch bugs are all one line long and none of them look broken.
4–6. The ones that lose enquiries
Submit the form and confirm a human receives it; check the failure path; confirm the success message appears. A form that silently fails is the most expensive possible bug because nobody reports it.
7–9. Redirects, links and 404s
Every old URL redirected to its closest equivalent, internal links pointing at final URLs rather than through chains, and a 404 page that offers a way forward.
10–12. The real-world checks
Load it on an actual phone, complete the primary action with a keyboard only, and check performance with a cold cache — lab conditions flatter you and hide exactly the problems users hit first.
13–14. Metadata and analytics
Unique titles and descriptions per page, canonical tags correct, and analytics recording on the pages that matter. Nothing here is difficult; all of it is easy to skip on a launch day.
Run the list as a list, with someone other than the builder ticking it. Self-review on launch day is how the one-line mistakes survive.