I have a ShareThis plugin user who has reported a problem with ShareThis. I've tracked it down to an incompatibility between ShareThis and Simple Tags. It appears that when Simple Tags replaces tags with links due to "Active auto link tags into post content" being set, it does the replacement on the permalink returned from the standard WordPress the_permalink() template tag. Basically, a slug that matches a tag gets replaced with a link, making an invalid URL. Here's an example:
I tried to create a quick patch for ShareThis by stripping HTML out of the permalink, but I wasn't successful, since it appears that Simple Tags applies the replacement after the ShareThis code is run.