If you attempt to select any text in yahoo mail it will select the whole message (and a large section of the UI).
1. Go to mail.yahoo.com
2. Attempt to select a word
4. Notice it selects everything.
This doesn't reproduce in shipping Safari 6.0.4 (8536.29.13). It's been going on in webkit nightlies for quite a while though.
Created attachment 201568 [details]
This is because yahoo.com sets -webkit-user-select: all for no good reason.