To reproduce: go to Peggy's profile & click the We ♥ Our Kids link in her sig (sorry Peggy! )
Had it happen in other signatures too, but Peggy's is the only one to come to mind right now. I'll edit this if I see more.
Wish I could give yas more info, but yeah, I'm fairly clueless about this one <_<
EDIT: Also happens on the following.
- VincentU's "Selling XenLanguages.com & XenInstallations.com!" link
- Shelley's "The Happy Place - Clickable Smilies Under Editor Control" link
EDIT #3 O_O: Not happening to all bold links either though.
EDIT #4: Just saw an occurrence of it happening outside of sigs - here. Clicking the link Brogan posted in that ...post, causes the exact same effect (or lack thereof)