When the "Start Game" sign was clicked, the pre-gen progress bar starts being visible.
During this time, it is still possible to generate a different card.

If you do generate a new card, the pre-generation itself is correctly aborted, however the progress bar keeps being visible.

This is especially confusing, when the "On card generation" setting is used for starting pre-generation. Players are generally used to the game starting automatically when the bar is full, however since the start game sign was never pressed again, the game will not start in this case.