Browser
ERR_CACHE_MISS A form could not be replayed from cache
What it means
The browser needed to resend a request that was not a simple "read this page," often a form POST, and it would not do that from cache without asking you. It is easy to read this as a failure. It is often a warning about a reload.
Where you usually see it
After you refresh a page that was the result of submitting a form, or after you press Back onto such a page. Some browsers show a confirm box instead of this badge.
What to try first
Do not reload until you know whether the first submit already worked. Open a fresh copy of the form, or go to the page that would show the result (an order list, a "thank you" page, an inbox). If you must resubmit, do it once and from the form itself.
When to stop
This is one of the easier ways to send a payment or an application twice. Treat the badge as a question, not as a broken site.