This is the sixteenth installment of the Fortnightly Topic Challenge described here, with topics suggested and voted on here. This fortnight's topic is reverse-puzzling (suggested by hexomino), and will span from the 19th of September to the 2nd of October. During this period, we will compile the list of questions with this tag and post it as an answer to this question.
In the meantime, please go and propose and vote on future challenges!
Everyone have fun, and happy puzzling!
Link to other Fortnightly Topics.