406 Error for post.php

I was in the process of posting when the next page showed me an error:

<i>Not Acceptable
An appropriate representation of the requested resource /blog/wp-admin/post.php could not be found on this server.</i>

I looked elsewhere on this forum for an answer, but it looks like some people had this problem solved almost immediately. One person said it had to do with HTML tags.

Does anyone have any idea what this problem is?