I've seen this happen before, though rarely. The recent change skankerror pointed to (which is also the topic james876 originally posted his to) may have made it happen more often, though. (I haven't really tried this or later revisions, yet.)
The worker issue occurs sometimes when workers have two adjacent buildings on their work queue. Then it can happen that, by the time the first building is done, one of the workers starts building the second building without actually moving into position - he just turns into the direction of the new building but keeps his position. I think this only happens when multiple workers are assigned these queued tasks and only if the worker actually building (not repairing) starts building the first building at a position that is between the location the first and the second building are to be erected at.
In this very case (screenshot posted by james876) there is only one building involved, though. I assume the worker who is building the building from a distance has previously been mining gold and had a queued task to build this building afterwards. Can you confirm this, james876?
