Unknown message runnung Tidy

Running Tidy over an HTML-document i get sometimes the following
message:

>>Tidy (vers 30th April 2000) Parsing "tmp.htm"
>>line 49 column 1 - Warning: html doctype doesn't match content

>>"tmp.htm" appears to be HTML proprietary
>>1 warnings/errors were found!

What is the cause for this message?

Thanks, Torsten