This position, composed by Garen Yacoubian of France in 1976, is an example of what one might call the pedantic school of series-helpmate. In an ordinary series helpmate, Black makes a specified number of moves to reach a position in which White can deliver instant checkmate. The only restrictions are that Black must not move into check, and may not deliver a check until his final move.
The pedantic version, however, also demands that every position reached after each move of the solution must be legal. The diagram position is a good example. It is a series helpmate (of the pedantic variety) in 11.
The idea is easy enough: Black must block the squares a2, b3 and b2 to reach a position in which Nc2 is mate. So let's start 1.fxe6, 2.exd5, 3.dxc4, 4.cxb3 and now we must stop to think. The obvious continuation is 5.bxa2, 6.Rb3, 7.g3, 8.g2, 9.g1=B, 10.Bd4 and 11.Bb2 - but that's not the answer. Look again at the position after 10.Bd4 and you will see that it is illegal: there is no possible White move that could have led to it. The knight cannot have moved from c2 (where Black would have been in check), and the king cannot have moved to a5 without having been in an impossible double-check.
We must go back to move four to correct matters. The right answer demands a little more delicacy: 5.b2, 6.Rb3, 7.g3, 8.g2, 9.g1=Q, 10.Qb1, 11.Qxa2 and White plays Nc2 mate.
The difference is that after 6.Rb3, the b6 square is now only covered once and the position is legal. White's previous move would have been with his king from b6 to a5.