I was using the mouse to drag around letters, not the keyboard. It was my first time playing and I was still wrapping my head around basic game concepts, so it's unlikely that I was interacting quickly.
Looks like the game can get itself into invalid states: https://imgur.com/a/3Wktj10. Don't want to spoil this board, but there are incorrect letters accepted as green here.
* edit: just saw that oceliker ran into the same issue.