In "Find the letters" levels there are tiles in the asked for word, that don't exist on the board. Here are some screenshots. As you can see, I found all the letters of the word, but the next word never showed up. I play on Facebook, in Swedish. I have encountered this bug with other letters too, but always with our special Swedish letters Å, Ä and Ö.
Thanks @Skarpa5 for posting, will ping Community Managers to have Studio look into the issue. Could you please specify what level exposes this situation, to help tracking this down?