e-mails disappearing from pop server but not from outlook

Does anyone know why an e-mail would disappear from online but yet still show up in outlook when checked from there? We had an instance today where 2 e-mails, when being checked online, were originally there and then when the browser refreshed they disappeared. The user did not delete them whatsoever. The person who uses outlook to check them still received them. Is this a freakish server error of the company that provides our pop account and has anyone ever heard of this happening before?

Thanks,
ShannonH