This has just happened again, so here's the details..
We have a shared content object with the html for the addthis button in it which is placed in a generic content block on a number of pages. The add-this code is just copied'n'pasted from the addthis site.
The valid code is:
|<div style="text-align: right;"><a href="http://www.addthis.com/bookmark.php?v=250&pub=chrisxtend" class="addthis_button" conf="[object Object]" share="[object Object]"><img width="125" height="16" style="border: 0px none ;" alt="Bookmark and Share" src="http://s7.addthis.com/static/btn/v2/lg-share-en.gif" /></a>
We had a problem with the erroneous code which I fixed up on the 14th, and then yesterday I added the shared content to a number of other pages.
It appears that every time I saved the page it added a new version of the shared content. In this case it appended:
|<iframe id="_atssh" style="border: 0pt none ; height: 1px; width: 1px; position: absolute; z-index: 100000; left: 0pt; top: 0pt;" src="//s7.addthis.com/static/r07/sh09.html#swfp=0"></iframe>
Each time it was added. This is part of the code that addthis renders from the script in the valid code.
A screenshot of the shared object history is attached.
As a workaround we could put the addthis button into a control rather than a generic content page, but I'm not relishing the idea of editing 700 odd pages...