An HTML only email was received and a rough conversion is below
When an email is received in HTML only, Email to Case Premium prepends the following message in the case description or comment. "An HTML only email was received and a rough conversion is below. Please refer to the Emails related list for the HTML contents of the message. For more information, please contact your Salesforce.com administrator."
In rare instances an email client sends an HTML email without a plain text version. A rough conversion to plain text is performed for incoming HTML-only emails. The Emails related list will contain a copy of the original message, including a link to the original HTML version, which users can refer to. If you don't have that related list, use the page layout editor to add it. If you do not see a link to the HTML version, verify that "Enable HTML Email" is selected (checked) under Setup > Customize > Cases > Email-to-Case
It may appear that a text body does exist when viewing an individual Email Message record because Salesforce also displays a rough conversion. This can be confirmed by inspecting the record's data using Data Loader or a similar tool.