If the Form Fails Validation, Show the User Which Field Needs Their Attention
If you really must validate on the server side and can’t do it on the client side (see #51, Validate Data Entry as Soon as Possible), then never send a user back to a form without telling them what to do next, and never with a generic message such as “there was an error.”
The user will have likely entered several different bits of data and they’ll need to get the context of the form back into their head again, once it comes back from server-side validation. The worst way to do this is by forcing them to scan the whole form again, looking for what they might have got wrong.
Having to scan the whole form again for errors is worse still for partially sighted users with screen readers (or other assistive technology) where the attention needs to be drawn to the specific field that needs to be amended. Highlight the problem (or problems) with the form and show the user where...
