The student in question had sent a panicky and confused email, explaining that she couldn't resubmit because she couldn't see a resubmit button. I had written a calming and detailed reply, only to realise (fortunately before sending the message) that she must have had another go after emailing me and that this time she had been successful.
It would have been nice if she'd then dropped me a quick line to say, "Panic over, all is now well," but she didn't.
 Which included a screenshot edited in PhotoShop.
 That she had submitted in the wrong format.