Enemy invades my board

not sure what causes it, but twice now i've had enemy units stay on my side of the board. the first time it was late game and not a big deal. This time it was early game where I need all the space I can get. you can't sell the unit. The champion was Jayce. I bought two more Jayces and it promoted him. I was able to put him on my board, but he attacked me when fighting on my arena. Tried to take a screenshot, but I guess I did it wrong.