If you are delegating some of your tasks, chances that you do that by email are pretty high. If this is the case you may face the same problem like Chris Sauve: As email responses come back and delegated tasks are completed you forget to check them off in OmniFocus where you, of course, track them.
Chris worked out a pretty powerful AppleScript to address the issue: It takes the senders of all messages you select in Mail.app and tries to match them to your 'Waiting For' context names. Any tasks found under a matched context will be offered to mark complete.
A very handy script provided you have your 'Waiting For' contexts organised in a certain way. Read Chris' post to understand the requirements to get it work.