The British Gas app that says 'it looks like there's something wrong here, are you sure' every time I input my meter readings, forcing me to go back and check I've not transposed a number (and numbers terrify me), which I haven't, so no, there's nothing wrong. As proved by the fact I don't have a bill for £3 billion.
Every form on the internet for displaying the 'form incomplete/you've done something wrong' whatever message and not telling you what, oh no, but cryptically encoding the source of the problem somewhere in the long form itself, perhaps a smidgeon of extra border around a text box, an inkling of pastel colour, or some vague signifier guaranteed to be lost in a form that has busied itself asking questions about stuff there's no reason for it know.