Or the second one might be that you have an ordered list and you have started a line in that list but instead of ending it with </li> you have ending it with <p> instead.
As Alex Bell response implies, these are reasons to know a little HTML. If not very familiar, then you can use
http://www.w3schools.com/html/default.asp
as a resource. It takes things step by step. The only caution is that not all things HTML that work for browsers work in epubs in general or for any specific ereader machine or program.