I have heard many people saying, “Hah! I beat Stockfish,” and one saying, “I am the best chess player ever! I beat Stockfish.” So I wonder if it is possible, just to know whether I should try to beat it. I tried to play it once; I barely played 25 moves.
The answer is that they either played it on some very handicapped mode, they took back A LOT of moves, or they are probably lying.
Stockfish 11 is currently rated about 3607, and that is in standard chess. Computer programs are typically even HARDER to beat for humans at even faster controls.
There is a reason elite humans have not played matches against computers for a long time: even the Carlsens of the world cannot beat them any more. According to the ELO winning expectancy calculator, there is currently a 745 point difference between Stockfish 11 and Carlsen, and per the chart, that gives the computer a 99.5439696% chance of winning.
Note: The ratings used above are from different rating pools, and thus, are not truly directly comparable, but used as a rough estimate of what would happen between the computer and Carlsen.
It's absolutely beatable, but not by an unassisted human. Anyone who claims to have done so is either lying or stacked the deck super heavily in their favor (e.g., by having Stockfish search only to depth 2). "Slow computer" isn't good enough - Stockfish 11 running on 1999 hardware would still have handily beaten Kasparov. You will need help from another engine to win.
Here's a recent game Stockfish lost to Leela Chess Zero.
[FEN "rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1"] [Event "Trillion-Node Throwdown II Tiebreaker (30|5)"] [Site "?"] [Date "2020.02.18"] [Round "1"] [White "Lc0"] [Black "Stockfish"] [Result "1-0"] [ECO "A41"] [GameDuration "01:13:46"] [GameEndTime "2020-02-18T11:44:09.048 PST"] [GameStartTime "2020-02-18T10:30:22.171 PST"] [Opening "Queen's Pawn"] [PlyCount "199"] [TimeControl "1800+5"] 1. d4 d6 2. Nc3 Nf6 3. e4 e5 4. Nf3 Nbd7 5. Bc4 Be7 6. O-O O-O 7. a4 c6 8. Re1 Re8 9. Ng5 Rf8 10. Nf3 h6 11. h3 Re8 12. a5 Qc7 13. Be3 exd4 14. Nxd4 Bf8 15. Bf4 Rb8 16. Bg3 b5 17. axb6 axb6 18. b4 b5 19. Bb3 Bb7 20. Re3 Ne5 21. Bh4 Nfd7 22. f4 Nc4 23. Rg3 g6 24. f5 Kh7 25. e5 Ndxe5 26. Ne4 Be7 27. Nf6+ Bxf6 28. Bxf6 Qd7 29. Ne6 Rxe6 30. fxe6 Qxe6 31. Qf1 Re8 32. Bxe5 dxe5 33. Rc3 e4 34. Bxc4 bxc4 35. Qxc4 Qd7 36. Qc5 h5 37. Re3 Qd2 38. Qc3 Qd8 39. Ree1 Qc7 40. Qd4 f5 41. Rad1 Kh6 42. Qd7 Re7 43. Qxc7 Rxc7 44. h4 Ba6 45. Rd8 Bb5 46. Kf2 Rf7 47. Rd4 Ba4 48. Rc4 Bb5 49. Rc5 Ba6 50. g3 Bb7 51. Ke3 Rd7 52. Ra1 Kg7 53. Ra7 Kf7 54. Rc4 Bc8 55. Rxd7+ Bxd7 56. Rd4 Ke7 57. Rd1 Be6 58. Ra1 Kd7 59. Ra5 Kd6 60. c3 Bb3 61. Ra8 Be6 62. Ra7 Kd5 63. Rg7 f4+ 64. gxf4 Bf5 65. Rc7 Kd6 66. Ra7 Kd5 67. Ra6 Bd7 68. b5 cxb5 69. Rxg6 Bc8 70. Rg5+ Ke6 71. Rxh5 Kf6 72. Rxb5 Bh3 73. f5 Bg4 74. Kxe4 Bh5 75. Ke3 Be8 76. Rb4 Kxf5 77. h5 Kg5 78. h6 Kxh6 79. c4 Kg5 80. c5 Kf5 81. Rd4 Bb5 82. Rd5+ Ke6 83. Kd4 Bf1 84. c6 Ba6 85. c7 Bc8 86. Rd8 Bd7 87. c8=Q Bxc8 88. Rxc8 Kd7 89. Ra8 Ke6 90. Ra5 Kd6 91. Re5 Kc6 92. Rd5 Kb6 93. Rc5 Kb7 94. Kc4 Ka6 95. Rc6+ Ka7 96. Kb5 Kb7 97. Rc5 Ka7 98. Kc6 Ka6 99. Kc7 Ka7 100. Ra5# 1-0
If these people insist they can still beat Stockfish 11 unaided, challenge them to prove it live. You can probably offer them odds of pawn and move and they'll still lose (they won't even draw). If they actually manage to win, it'll be big news. Here's an example of a claimed win from 2011, at a time when computers were already unbeatable by humans. As ChessBase put it, "Believe it or not? Not, of course. Definitely not." Pointedly, the guy making the claim was later convicted of fraud.
People who say they beat Stockfish did it at a low level, with a fast time control so Stockfish could not do its best, and perhaps also on slower computers.
Can Stockfish be beat at high level on fast computer with long time control? Of course, but it would likely take another computer program to do it.
Remember that Stockfish was rated at 3388 as of July 2017 and would be even stronger now.
I would say it is practically unbeatable by a human player when running according to the TCEC hardware, but theoretically, it could be since chess has so many possibilities that we only completely understand it when only 7 pieces are on the board(Endgame Tablebases). Another fact is that these tablebases were created by supercomputers. I am certainly sure that it could be beaten by non-human players especially Neural Networks.