How could this happen? (Nasty hard.)

Solution:

Aha a terrible blunder by yourself in the top one there is a pawn on e6 but in the bottom one the pawn is on e7!

Doen't matter - the whole proof-game is perhaps needed to to prove the legality of the pos. 100%, but anyone who's tried retroanalysis will find this pretty simple ... for one, it is trivial that the B on h8 is a promoted piece - I don't see any difficulty anywhere.

Have you seen the one that goes something like: 'the 105th move has just beeen played. Find the last 91 moves.' That one is really tough, even with the help of a retroanalysis program it took me a week ...

Two books to start one with on retro-chess: Both by Raymond Smullyan:

1. The chess adventures of Sherlock Holmes.

2. Arabian Knights.

Also try 'the retrograde analysis corner':

http://www.janko.at/Retros/index.htm

That wouldn't accrue in a real game.

I agree this game profile would be between 5 year olds with dad's chess set.