Error?

What's wrong with this position:

It's an illegal position.

I suppose with the capture it's a legal position.  Legal, but 7... Nf8 looks like a better move.

Now, my friend, your illegalness has been refuted. 

Yes it has.  In fact, it could have been any capture except for a pawn and the dark squared bishop.

Most people already understand this, but it can't be a pawn since the white king would be in check.  It can't be the dark squared bishop since it'd have to move to e7 from f8, which would mean the white king can't be on d6.  No other piece can start on a square that checks the white king.

The queen has to start on e1 - e4 or e8, or she can be on f7-h7 (8 squares).  The rook can start on e1 - e5 or e8, or f7 - h7 (9 squares).  The knight can start on c6, d5, g7, or g8 (4 squares).  So we have 21 possible positions that could result in this mate.

Also, this is much simplier diagram.  :-p

Oh, the cxd7+ brings in more possibilities, including the light squared bishop!  It looks like if we have a capture on the first turn, we have more possible positions than I care to consider.

Alright, well, here's an easy one:  Is this a legal position?

I'm going to have to try to make a harder one soon.

Yes. A white pawn was underpromoted to a bishop. 

Here is the original legal/illegal thread-

http://www.chess.com/forum/view/general/legal-and-illegal-positions