Thanks for reporting that, blackrockrose. I had this happen to me once, a couple of days ago. I can't remember how I got things working again - maybe just clicking on the rogue letter on the rack. This was in a daily 7-by-many. I'd also be interested to know if this is happening to other people.
It's probably an accidental by-product of the changes I recently made to try to improve play on touch screens. I imagine it only affects the 7-by-many puzzles, because in these puzzles the letter played is not meant to move up to the rack at all, since it should remain available for playing again in the same word.
You mention clicking on a letter. I presume that means you're using a touchpad or mouse. Or does your Macbook have a touch screen? You could try using the keyboard to select the letters and see if the same problem occurs.
I'll see if I can work out how this is happening, and put a stop to it.