We took over an older company site, and during the handover we put test data into its contact form. The page thanked us. Nothing turned up in the inbox.
The form was posting fine. The thank-you message was written straight into the page, so it showed up whether or not the message was ever delivered. Outbound mail had broken somewhere behind it, and nobody could say when. There were no errors to find because nothing was erroring: the site was quietly taking enquiries and dropping them.
The cost was the part you can't get back. We had no way to tell the owner how many people had tried to reach them.
Two things are standard for us now. The success message only appears after the backend confirms the message was actually handed off. And we alert on silence, so if a form that normally gets enquiries gets none for several days, someone gets pinged. Most monitoring watches for errors, and the worst failure we've seen on a website produced none.
If you own a site, the five-minute version is this: send yourself a message through your own contact form today, and check that it lands in an inbox a human actually reads.
Probably worth knowing when you last tested yours.