Mega Error by Electric Putty seeks to solve the problem of hidden errors across tabs in the publish form. In the forum post where Rob Hodges announced the free add-on he described it like this:
You have a channel with lots of fields. You’ve spent ages making a beautiful custom publish layout for your client to make their life easy. You’ve separated your fields across multiple tabs and everything looks great. Your client then creates a new entry and leaves a required field empty. EE captures the error and adds a warning, but the field is on the second tab and the client can’t see it. They have no way of knowing there is a problem; they don’t even know the entry hasn’t been saved.
Mega Error displays a giant message at the top of the publish page with a summary of errors and then an error with each field across all tabs.
(Image from Mega Error product page)
Mega Error is free and available now only for EE2 from the Electric Putty website.