One of our client recently had an annoying comment posted on his site. Even more annoyingly, the commenter had posted a fake email and clicked Subscribe to Comments. Now this client runs a pretty busy client section: 200 comments per post is not uncommon.
Result of the fake address passing: over 200 bounced emails in his inbox. Richard tried to remove the commenter's subscription in the admin section of Mark Jaquith's Subscribe to Comments (WP Plugins). It didn't work.
We tried to block the user from receiving any comments. Didn't work.
Clearly there is an issue between WordPress Subscribe to Comments and WordPress 2.9. Easy enough to fix. But we have about five popular plugins right now in active development and adopting another step-child would take us away from our existing work.
What did work for stopping the bounced emails, we simply removing the commenter's address from the email.