It still happens. Someone writes three careful paragraphs describing what they need, presses send, and the page returns blank because a phone number was in a format the validator did not like.
They do not retype it. They leave, and the business never learns the enquiry existed.
Validate where the mistake is
An error at the top of the page, pointing at a field somewhere below, asks the visitor to do the matching. It gets worse the longer the form is.
The message belongs at the field, in words that say what to do rather than what went wrong. Enter a date after today helps. Invalid input does not — it is a description of the validator’s feelings.
For a long form, a short summary near the button earns its place, because that is where the person is looking when nothing happens. But a summary that repeats a single error already visible two lines above it is noise: it should appear when it adds something, not on principle.
Never throw away input
Whatever happens, what the person typed comes back. This is the rule, and it holds even when the failure is on your side — a timeout, a rate limit, a spam check that misfired.
The spam check is worth its own thought. A form that silently discards a submission it distrusts is optimising for the operator and against the visitor who has done nothing wrong. If a submission is refused, the person must be told in a way they can act on, and their text must still be in the boxes.
Be generous about format, strict about meaning
A phone number typed with spaces, dots or a country code is the same phone number. Rejecting it teaches the visitor that the form is fussy; accepting it and normalising it afterwards costs a couple of lines.
Meaning is different, and worth being strict about — a required field that is genuinely required, a date that cannot be in the past. Those rules stop bad data entering the business. Format rules mostly just stop people.