I have created a document that has one ordered list with seven steps and six images. After I create the post, if I look at the HTML, I see sometimes up to four ( ! ) extra <ol></ol> or <ul></ul> elements in between paragraphs. Sometimes an open ol or open ul tag will appear in the middle of a line, and no matter how many times I delete them, and either Save, Save and Continue or Publish, the tags return immediately. Sometimes it happens even if I just switch from the HTML to the WYSIWYG editor!
I am using Firefox 16 on Win7 but it has happened on other platforms and different browsers as well, so I don't think it's browser- or OS-specific.