Steps:
- Select a challenge that you have not already played/downloaded and click ‘Details’ button
- Click ‘Download’ button
- Deploy your fleet and fight the battle (loose it)
- Click 'View statistics’ button
- Click ‘Continue’ button
- Select the same challenge again (do not leave the challenge screen) and click ‘Details’ button
- Click 'Download’ button (–> crash!)
Please note that at point 7. the button is called ‘Download’. Usually, challenges that are already downloaded have a ‘Fight’ button instead.
Repeatable with any challenge, as long as you did not play/download it before (see point 1.).