First bug I've found on lichess (it doesn't affect anything)

lichess.org/editor/8/8/8/8/8/2n5/1QQn4/QK2k3_w_-_-_0_1?color=white

The usual impossible position. But this position can be played out with a computer and probably a friend. (lichess.org/Xaip9Ckd)
What is the reason for this? The point is that this position is also considered possible in the syzygy tables.

syzygy-tables.info/?fen=8/8/8/8/8/2n5/1QQn4/QK2k3_w_-_-_0_1

That's funny I would never get into this position, but the site itself recommends to see this position as the longest win of the weakest side in the ratio KQQQ vs KNN (0-1). It is unlikely that this bug can be fixed, so I think people have found similar mistakes before. It would be interesting to see such positions in more realistic ratios without two or more queens)

I don't understand, what is the bug? What is the incorrect behavior, and what behavior would you expect to see instead?

two knights are both preforming a check on the king.

This is impossible because a king would either have to stay in check for more than 1 move (illegal) or both knight would have to move in the same turn (also illegal)

I feel like that's on the person who set up the position. If lichess is explicitly told to start a game from that position, I don't see any reason why it shouldn't.

Are you saying "if I ask lichess to start a game from a position that cannot be legally reached, I expect lichess to prevent the game from starting, rather than go ahead and start a game?" If so, why?

@PrydunAndrii2004 said in #1:
> What is the reason for this? The point is that this position is also considered possible in the syzygy tables.
Syzygy tables have some impossible positions, that doesn't mean they're all possible.

> It is unlikely that this bug can be fixed,
It's not a lichess bug, it's a tablebase mistake, so let's not discuss that here.

@corvusmellori said in #4:
> If lichess is explicitly told to start a game from that position, I don't see any reason why it shouldn't.
It shouldn't for the same reason that it doesn't allow to start other impossible positions, like the ones without one or both kings, and ones with pawn(s) on first rank.

@lakfish said in #5:
> It shouldn't for the same reason that it doesn't allow to start other impossible positions
I think there's a big difference between this position and one with no kings. In this position, the rest of the chess game can proceed absolutely normally. In a position with no kings, or a white pawn ok the eighth rank, or with both kings in check, that's not the case. There's no reason to avoid starting a game with a king in check from two knights, or with White having fourteen bishops, or with Black having eleven pawns. (as it happens, I also think it's perfectly fine to start a game with a white pawn on the first rank, but I admit it does make the "pawns can advance two squares on their first move" rule and en passant rules bit confusing).

Maybe it could be some tool to check whether the position is legal? Then everyone
(especially beginners) would be sure whether they analyze a position which can occur
in games or they consider a position impossible to reach. In case when a position were
marked as illegal, one could treat it as a small exercise: explain why this position is illegal.

@Marcin2 said in #7:
> Maybe it could be some tool to check whether the position is legal?
I think in general this is probably not easy for a computer to decide.

For example, here are some positions that cannot be legally reached (but it's not always simple to figure that out)
2bqkbNr/rppppppp/p1n5/8/8/P7/NPPPPPPP/1RBQKBnR b Kk - 0 1
8/5kn1/6p1/8/8/8/P1P5/KB6 w - - 0 1
8/8/8/2K1k3/8/8/8/B7 b - - 0 1

@corvusmellori said in #8:
> I think in general this is probably not easy for a computer to decide.
>
> For example, here are some positions that cannot be legally reached (but it's not always simple to figure that out)
> 2bqkbNr/rppppppp/p1n5/8/8/P7/NPPPPPPP/1RBQKBnR b Kk - 0 1
> 8/5kn1/6p1/8/8/8/P1P5/KB6 w - - 0 1
> 8/8/8/2K1k3/8/8/8/B7 b - - 0 1

I see no reason why the first cited position would be illegal. It can be reached after 1. Nf3 Nf6 2. Nh4 Nh5 3. Nf5 Nf4
4. Nh6 Nh3 5. Ng8 Ng1 6. a3 a6 7. Nc3 Nc6 8. Rb1 Ra7 9. Na2 and black is on move.

It is legal if it is from position