Tactics
Underpromotion
Occasionally the queen is worse than a knight, because a knight gives check and the queen does not, or worse than a rook, because the queen would stalemate them. Underpromotion is rare enough that most players never do it in a real game, and the positions where it is right are unforgettable.
How to spot it
Two triggers, and they are almost the whole story. If promoting to a queen leaves the opponent with no legal move, look at the rook. If a knight on the promotion square would give check or fork the king, count that line before the obvious one.
The positions
Easiest first. Every one is from a real game, and the rating under each board is what it costs the players who have tried it on lichess.
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1. White to play Play it out →Answer
53.h8=N+ Kh5 54.Rh7#
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2. White to play Play it out →Answer
45.d8=N+ Kf6 46.Rf7#
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3. White to play Play it out →Answer
48.f8=N+ Kh8 49.Bd4+ e5 50.Bxe5+ Kg8 51.h7#
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4. Black to play Play it out →Answer
52...g1=N+ 53.Kh4 Rh2#
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5. White to play Play it out →Answer
41.Qxg7+ Kxg7 42.e8=N+ Kf7 43.Nxd6+
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6. Black to play Play it out →Answer
27...g1=N+ 28.Kf1 Qxd5
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7. Black to play Play it out →Answer
59...fxg3+ 60.Kh1 g2+ 61.Kh2 g3+ 62.Kh3 g1=N+ 63.Qxg1+ Kxg1
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8. White to play Play it out →Answer
59.g7 Re3 60.g8=N+ Kf7 61.Nf6 Kxf6 62.h8=Q+
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9. White to play Play it out →Answer
48.f8=N+ Rxf8 49.h5#
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10. White to play Play it out →Answer
38.d8=N+ Rxd8 39.R1b6+ Rd6 40.Rxd6#
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11. White to play Play it out →Answer
31.Rxf8+ Rxf8 32.Qxh6+ gxh6 33.g7+ Kh7 34.gxf8=N+ Kh8 35.Rg8#
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12. White to play Play it out →Answer
13.dxc8=N Qxd5 14.Ne7+ Kh8 15.Nxd5
Reading about a pattern is the easy half. Motify finds the positions where you missed one, by running your own lichess and Chess.com games through Stockfish, and puts each of them behind two or three puzzles on the same idea so you meet it having just found it three times. See how Motify works.
Positions from the lichess puzzle database, released under CC0.