There appears to be a bug that was spotted in the iPhone 3.0 Spotlight, this was discovered by Matt Janssen. It would seem that the search feature keeps emails in the cache/index once they have been erased from the inbox and eliminated from the trash.
The video which you can view shows Matt Janssen stating that he found emails which he erased a few months ago. Firstly if you were to tap on an erased email in a search result Mobile Mail would be launched, it would then crash, but if you were to do this process again it will show the email, even if it was in a bugging manner.
This bug allows you to pull messages that are quite a few months old only if you know the subject line. Engadget were informed recently that the bug may be fixed in iPhone 3.1. Apple’s next iTunes/iPod event is said to take place in the week of 7th September, so maybe this is when we will see the release of iPhone 3.1, now on Beta 3.
Source – theiphoneblog.com