Nice, bought to replace 'chess tactics puzzles' the addition of rating changes sold me.
Too late did I notice that there is no share fen button. I use that for harder puzzles when I want to understand why a given solution wouldn't work. Mostly just share to Chessis.
Bugjail was mentioned in a thread about a xor bloomfilter alternative. Or at least that's where I saw it first mentioned. My bet is it triggered op's posting.