Illegal PGN Puzzles
Double check doesn't help - the mate is just an illusion :
Read the OP. Kings are restricted to normal movement.
Here's a slightly different puzzle, and probably more sensible. Legal moves must be made, but the starting position may be illegal. Heres a very easy mate in 1, but I may try to make some harder puzzles later. Unfortunately the puzzlemaker does not allow illegal positions so I give a diagram.

Don't worry about all the grinchs who say this is stupid, I'm not into this stuff myself but there are other people who are. Occasionally "illegal" or "impossible" puzzles are fun, you might consider forming your own group if you want to find other members here who actually like this stuff.

Here's a slightly different puzzle, and probably more sensible. Legal moves must be made, but the starting position may be illegal. Heres a very easy mate in 1, but I may try to make some harder puzzles later. Unfortunately the puzzlemaker does not allow illegal positions so I give a diagram.
1. Nb6#
Great puzzle. No matter where Black moves, all three kings will still be in check.
This forum is dedicated to posting puzzles that arise in messed-up positions caused by playing with PGN files. In these puzzles, pieces can go anywhere, pawns can promote on the first move, pieces can move past other pieces,... The only exceptions are the following (these were only put in to reduce the number of solutions to these puzzles):
1. The king is constrained by the normal rules of chess.
2. No piece can capture a piece of its own color.
Here is an example puzzle:
And here is the solution, as well as the rest of the game: