Title. It happens randomly, and I won't be able to buy any units, or even see which units are able to be bought, because I'm stuck on the "Sell for X gold" screen instead. I've found that the only way to clear it is to pick up a unit on my bench and place it back on the bench.
I just had a game where I sold my first unit and was going to buy 3 from the selection, but the "sell for X gold" banner stuck. Since there were no units on my bench to pick up and put down again, and I could not buy any units because of the banner, I was 100% stuck, and watched the minion rounds kill me. I eventually left the game and reconnected (which solved it), but ew.