We can't tell what the last pawn move was. It could possibly be h2xRg3 or g7-g6:
An original retro problem #5
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Yeah I went to sleep and as I woke up, I knew I made an error at the last moment.
Here is the corrected diagram. Now, it should take you but a moment to post the solution :)
I'm starting to run out of ideas, so the next retro puzzles are probably going to be easier...
What was the last pawn move?
(Hint: FEN is important here)
EDIT: there are two solutions. corrected diagram in post #3.