If you hit a certain speed (I was at 53 when this happened) The background turns green and you are unable to collect anymore coins. Refreshing fixes this.
Please get rid of the cutscene when you combine elements; it takes away from the fluidity of the game. Maybe just have a popup or a notification on the side.
Found a bug :O If you try to build something but click the build button again then click cancel you can move around while your trying to build. Good game though :D
Fixed!