Calibre's metadata window:
Screen1:

(Click to enlarge.)
Button order: 1) Next 2) Cancel
Screen2:

(Click to enlarge.)
Button order: 1) OK 2) Cancel
The position of Cancel button is swapped in consecutive screens. Because of that I clicked the wrong button (OK) thrice -- and I had to start from scratch every time.
So, please consider putting the "next" and "ok" buttons to the right, and "cancel" to the left in both screens.
Thanks for the fantastic program!