This position was "composed" by Richard Reti and published in 1921.
2...Ka6-b6 3 Kf6-e5 and 3...Kb6xc6 4 Ke5-f4 h4-h3 5 Kf4-g3 or 3...h4-h3 4 Ke5-d6 h3-h2 c6-c7.
White to move.
It is obvious that his king cannot catch the h-pawn, so it must promote.
It is also obvious that his king is too far away from his own pawn to help it get to its promotion square (c8).
The wonder is that two "cannots" make one "can"! 1 Kh8-g7 h5-h4 2 Kg7-f6 and now:
2...h4-h3 3 Kf6-e7 h3-h2 4 c6-c7 Ka6-b7 5 Ke7-d7 draws, as does
This position arose in a 1991 British Championship game, Jim Plaskett-Jonathan Parker.
Therefore the g4 pawn is both pinned and not pinned at the same time!
Black's g4 pawn was pinned by the Ra4.
White has just played f2-f4.
Therefore the g4 pawn is no longer pinned and could move g4-g3.
BUT the g4 pawn IS still pinned, because the "en passant" capture, g4xf3 (which would normally be possible), is illegal because it would leave the black king in check.