Positive that you saw an interesting comment from someone on Twitter but can't find it in their feed? Tweleted lets you search by username to find deleted comments.
The service works by comparing results from Twitter's search feed with comments on public feeds. If it spots a difference, it lists those results. That approach means it's not likely to find long-deleted tweets, and is unlikely to locate posts from people who have set up Twitter for maximum privacy.
Tweleted certainly isn't perfect -- I've only knowingly deleted a couple of tweets recently, and neither showed up. The site's creator also points out that it can flag comments as deleted even when they're actually still visible (though that's easy enough to double check by looking at the actual feed for that person). Nonetheless, it's a possible resource if you want to track down an elusive Twitter comment.