invisible mail stuck in junk folder

Hi,

The folder list on the left of the ajax web interface shows 1 mail in Junk, but there's nothing there. "Empty junk" doesn't clear it either.

This mailbox is access from the web client, and via IMAP from an iPhone, and I can't see it on the phone either.

Any ideas gratefully received!

Thanks...

EDIT: this is a ZCS install on a VPS I administer myself