This bug is bad: It teaches that there`s only one mate on 4 in this King + Rook x King elementary position, saying that Kc3 on first move is wrong, but Kc3 also gives mate in 4, so it`s a solution as well.
http://en.lichess.org/training/25572
There are many other kind of bugs, like puzzles that say immediately that you failed, even before you make a move. In this case, you can open the link and solve it without that message. The instant failure message appears only during the training.
And of course, the most ridiculous type of bug: The answer that is considered the right one is a mate in 6, and I found a mate in 5, with Qxf7 first, instead of RxR first, 100% confirmed: http://en.lichess.org/training/49254
http://en.lichess.org/training/25572
There are many other kind of bugs, like puzzles that say immediately that you failed, even before you make a move. In this case, you can open the link and solve it without that message. The instant failure message appears only during the training.
And of course, the most ridiculous type of bug: The answer that is considered the right one is a mate in 6, and I found a mate in 5, with Qxf7 first, instead of RxR first, 100% confirmed: http://en.lichess.org/training/49254