Since I upgraded to Mountain Lion, every time I open Safari I got: "Safari 6.0 has not been tested with the plugin LastPass 1.73.0". Except I wasn't using LastPass in Safari and 1.73 is a really old version. Safari seemed to work fine, but it was annoying, even though I don't use Safari as my primary browser (I mostly use Chrome, and Firefox for LogMeIn). In case anyone else out there has the same problem, I'm posting how I solved it.
I tried to track it down several times with no success. Safari > Preferences > Extensions didn't show anything. I couldn't find anything useful searching on the web. Searching for "lastpass" files didn't uncover anything. I tried deleting extensions.plist and the Safari cache.
Finally I saw a suggestion (for an unrelated problem) to download LastPass and run the uninstaller. The closest to 1.73 I could find was LastPass 1.75 on the LastPass old downloads page. I opened the dmg and run the uninstaller, and that solved the problem. Now Safari opens with no error messages. I still don't know where the extension was lurking though! (I even checked the Trash after running the uninstaller, but couldn't see anything.)