Sold the unit I replaced, instead of what was currently being held by my cursor.
1. Have a unit on the field (Ashe in this example)
2. Purchase a unit (Garen in this example)
3. Swap the unit you just purchased (Garen) with the one on the field (Ashe)
4. BEFORE setting the unit (Ashe) down, sell it by dropping it in the sell area at the bottom of the screen.
5. (Garen) will be the unit sold, instead of the unit you dragged into the sell area (Ashe).
This would be a HUGE quality of life fix, and I'm sure many out there have fallen victim to this already.