Originally posted by SwissGambit
The first thing to do is re-count: Pf2 to a7 is only 5 captures. There are two more captures (axb and dxc3) for a total of 7. 7 captured + 9 showing = 16, leaving no 'extra' captures.
The second thing to do is figure out how to get Bc8 home. Bb1 leaving means b2-b3 and b4-b3 and a2xBb3. I can't do this right away because of a7. Something was captured there, but it was not the a7-pawn, because that shuts wBb8 in. Pa7 couldn't have been captured anywhere else on the a-file, so it promoted.
One try is White retracting d2xQc3 to let the Black a-pawn unpromote. This means killing Bc1 at home, since b2-b3 is White's next retraction. Bb8 is therefore promoted, and it must have been the h-pawn [the only one not accounted for]. Ph5 must make two captures to let it through, but this is a bust - 2 captures needed for the 3 Black pawns, plus one on c1, plus 10 White units showing = 17.
This is fixed by putting the dark-squared Bishop back on the board immediately with ...f6xBe5 [this cannot be done with gxBf6 which shuts in the promoted Bh8]. The solution is that a White Bishop was captured on e5