There used to be a bug that caused some merges to not be removed from the pending list even though they were completed. This has been fixed, but there are still old completed merges on the list. If you can't remove them, you can send me a list of them (with both IDs) and I can remove them.
I am gradually going through all of these merges, site-wide, starting with the oldest, but it will be quite a while before they're all gone. I'm currently done up July 2011, and am working on August 2011.