Nxf2 is an inaccuracy because Bxf2+ forces the king to move which prevents castling, and in both cases both pieces can be saved. Preventing castling is a better advantage than the other move, that's why it's inaccurate.
question about why this is an inaccuracy
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https://www.chess.com/analysis/game/live/19695128553?tab=analysis
8....NF2
i'm thinking it's an inaccuracy, because the opponent gets to eventually long castle and develop his pawns while chasing my knight off.. Is that right?
Whereas 8..BF2+ prevents him castling, and the bishop can move off in one go.