Comment 4 for bug 971757

I found the same thing. I used an activity that does not appear in the Activities list of any of my Categories, including the Unsorted category. I tracked the activity for a few minutes, stopped tracking and then added a category. I could not afterwards remove the category.

It appears that once a tracked activity, which was previously Unsorted, is placed in a named catergory, you cannot revert that tracked activity to Unsorted. This happens regardless of whether the category is tagged during or after tracking.

A workaround is to start tracking a new instance of the Unsorted activity, stop tracking it after about a minute, and then replace its start and finish times with those of the irrevocably categorised instance of the activity. Delete the categorised tracked activity.