When I go to the post entry page, if I switch from HTML to Rich Text and then back to HTML, suddenly this code appears in the text area of the entry:

<script src="chrome-extension://hhojmcideegachlhfgfdhailpfhgknjm/web_accessible_resources/index.js" type="text/javascript"></script></pre>

(Using textarea because I don't know of a better way to post it unedited.)