This happened just today with kiyoshi91 and soulshot. We won the round and then soulshot left the game, hes 4 x lvl7s got given to kiyoshi. But since she already got all 4, the mills dropped on the ground as items.
The bug was, that she still got the WPS from those 4 mills even they were lying on the ground as items. We noticed it after endless waves RIPped everything, except 1 hero. Kiyoshi still got -wps of 44.000 and solid income. There's couple of pictures showing the income over time.