Whilst playing the game one day, somebody sent me a text. So I went to minimize the app to view it. When I opened the game again, this happened.
And it never went away until I finished the level. Obviously it made it impossible to see how many moves I had left. Also I couldn't see what else I had to collect to finish the level. The score gives away the fact that I had collected everything by that stage, but it made life very difficult for about 10 turns.
What I think happened is that instead of minimizing it, it accidentally pressed the shovel booster (I had used all mine up by that stage) and was offered a way to get more. I did NOT close the offer. I then minimized the app and answered the text. This then happened. I tried it again, but this time closing the offer and it worked OK.
That is about the best way I can explain it and it has happened a few times under similar circumstances, so it definitely was not just a one-off thing.