Deploys that were invisible to everyone who had visited before

Symptom

A fix went out, verified in a private window, confirmed live. The client still saw the old behaviour, and so did anyone who had opened the site that week.

False lead

We spent a round hunting a build problem, because the served file on the server was demonstrably correct. It was — nobody was asking for it.

Cause

Stylesheets and scripts carried no version in their filenames and were being served with a seven-day cache. Returning browsers had no reason to ask again, so every deploy landed for new visitors only.

Fix

Code assets now revalidate on every request; only genuinely immutable files — fonts, images with hashed names — keep a long cache. Verified from a cold and a warm browser after each release.

—Verify in the state your users are actually in. A private window is the one visitor who has never been to your site.