Many times while working on recent comment sections on tabbed sidebar widgets or separate code to show recent comments with Gravatar and excerpt linked to the post, there are some issues we faced in wordpress
Mainly, issues were, comments titles were same for all and linked to same post even comments were from different posts and all comments were different. Also some comments shown with the link of the post which don't have a single comment on it, Finally I got the solution on http://stackoverflow.com
Please check the link or you can google the topic: "Display recent comments with gravatar and excerpts?"