I thought I was going crazy but I'm convinced now that it's not me. This is specific to Calls on Task List. I will clear / complete a Call on my Task List and a few weeks later it re-appears. Anyone else ever experience this?
I recommend running all of the steps listed for ActDiag, with the exception of 'Rebuild Sync Options' and 'Remote Accounting Link IDs'. You'll definitely want to run the 'Fix Known Activity Data Issues'. At this point, do not continue through the 'Importing to a New (or Empty copy) database'.