user16232: There are some fairly obvious ways to set up a chess board that cannot possibly happen:  both kings simultaneously in check would be one.
There are some positions that seem impossible in a normal game... for example, one side has all of it's original pawns in their original location and the other side has NONE of its pawns remaining - but with both sides displaying all pieces in their original positions (perhaps knights can do this, I don't know.)
However, seems like there may be some impossible positions that are not at all obviously difficult. 
The question is if there are any known legal positions (preferably ones that  a normal human might think are the result of a game) that cannot be played into?

user8201: Hi and thanks for the question. The words "legal" and "illegal" are actually defined in the FIDE rules. Basically, a legal position is one which can be reached by a sequence of legal moves.
So, with this definition, I think that you are asking: are there illegal positions which aren't obvious? The answer is "many" Welcome to retrograde analysis! :D
There is already a chess.stackexchange question on this, so I suggest we continue the discussion there: Are there any illegal positions that are difficult to spot?

16232: <p>There are some fairly obvious ways to set up a chess board that cannot possibly happen:  both kings simultaneously in check would be one.</p>

<p>There are some positions that <em>seem</em> impossible in a normal game... for example, one side has all of it's original pawns in their original location and the other side has NONE of its pawns remaining - but with both sides displaying all pieces in their original positions (perhaps knights can do this, I don't know.)</p>

<p>However, seems like there may be some impossible positions <em>that are not at all obviously difficult.</em>  </p>

<p>The question is if there are any known legal positions (preferably ones that  a normal human might think are the result of a game) that cannot be played into?</p>
 8201: <p>Hi and thanks for the question. The words "legal" and "illegal" are actually defined in the FIDE rules. Basically, a legal position is one which can be reached by a sequence of legal moves.</p>

<p>So, with this definition, I think that you are asking: are there illegal positions which aren't obvious? The answer is "many" Welcome to retrograde analysis! :D</p>

<p>There is already a chess.stackexchange question on this, so I suggest we continue the discussion there: <a href="https://chess.stackexchange.com/questions/15210/are-there-any-illegal-positions-that-are-difficult-to-spot">Are there any illegal positions that are difficult to spot?</a></p>
