I’m currently experimenting with temporary collections of sentences to be reviewed during the day, which I delete on the following day. I have thus discovered that you only get one warning before deleting a custom collection, whereas you get two warnings if you simply want to reset your progress in a collection. Given that the latter action is a lot less destructive than the former, shouldn’t it be the other way around?
Also, I think it would be helpful if the warning message contained the name of the collection to be deleted. I very nearly deleted a very valuable collection the other day.