Here is The Ruins of Murkhill list of the Unwritten/Unspoken Rules of the Original Dungeons & Dragons.
0. The Referee's Word is Law in the Game
1. Never split the party
2. Don't Be A Jerk
3. The statue is never just a statue
4. It Might Just Be A Mimic
5. Never confront characters with threats they cannot either defeat or avoid.
- 5b. Never confront characters with threats that have no solution
6. Assume players have taken reasonable actions.
7. Never let players ignorantly take a substantial risk.
8. Never plan to take the players’ freedom or stuff to support your plot. (Better yet, don't have a plot!)
9. If it is not written on your character sheet you do not possess it.
10. If you don't say it, it didn't happen and you didn't do it.
11. "Are you sure you want to do that?" is code for "What you just said you are going to do, just might be pretty stupid."
12. The Dice Giveth & The Dice Taketh Away
13. Expect disaster and roll with the punches.
14. The Party Will Be Mislead and Deceived
15. Always watch your teammate’s back. Don't let anyone sneak up on them.
16. Anything said by the players in or out of character can and will be used.
17. Thieves in the game should be in front leading the way.
18. A good Referee is not adversarial, a good Referee is impartial.
19. All PCs should have a chance to shine.
20. The golden rule for Players is "Thou shalt not hog the spotlight."
21. Everything has consequences.
22. Always as a priority, take out the opposing spell casters.
23. Always check the bodies.
24. There is no such thing as too much rope.
25. Timid players and over confident players are the two prime causes of TPK's, a party consisting of these two types will die.
26. Missile weapon using characters should always have a melee weapon attack.
27. Melee weapon characters should always have a missile weapon attack.
28. Don't forget the options of negotiating, hiding, running, etc. Sometimes the one thing you don't want to do is fight.
29. Pay close attention to descriptions.
30. Make sure what you kill stays dead.
31. Not all prisoners should be "rescued" and set free.
32. Talk to NPCs.
33. Don’t use area of effect spells in close quarters.
34. There is no such thing as an insignificant detail.
35. Never push a button, pull a lever or it's equivalent without knowing what it does.
36. Never eat or drink something in a dungeon, unless you have first verified it is safe.
37. Thou shalt not touch someone else’s dice without permission.
38. Thou shalt not be a Rules Lawyer.
39. All player die rolls should be in the open.
40. Many, but not all, Referee die rolls should not be in the open, because that will give players information that they have no way of knowing before they take the action.
41. In a face to face game, it is both expected and assumed that you bath regularly.
42. Unless you're actually IN a fishing village, any humanoid NPC found fishing is 99% likely to be something else in disguise, and often it will be a dragon.
43. Nothing is ever THAT easy.
44. Sea travel is never safe.
45. The Referee runs the kind of game he would want to play in as a PC.
46. Do not steal from or attack other PCs.
47: Sometimes the monsters should be low on HP too.
48. If you are playing in a home or anywhere snacks are brought and you are eating them, you should also be bringing them.
49. Hire hirelings and torchbearers - especially in the dungeon they are you carts and wagons - also they're your meat shields (cons to this) and light sources.
50. Players map the dungeon—the referee only describes.
51. Most doors are stuck - thieves (or strength checks) open them, the two most common options.
52. Wandering monsters every 10 minutes - dawdle and die.
53. Player ability over character stats.
54. The 10' pole is mandatory gear.
55. Ask detailed questions: What do I see/smell/hear?
56. Encumbrance matters - overloaded parties move slow and fight poorly.
57. Declare actions before initiative or rolls - no take-backs/no redo's.
58. Reaction rolls first - monsters might negotiate, flee, or surrender or do something else rather than fight.
59. During combat/encounters, morale checks for monsters and hirelings - they break and run.
60. Puzzles and traps reward thinking, not dice.
61. Some encounters overmatch the party - but can be avoided or tricked.
62. Cursed treasure tempts the greedy.
63. The easy win is always a trap.
64. Environment is your ally: chandeliers, benches, high ground.
65. Most wandering monsters have no treasure - skip them if you can (learn the exceptions).
66. Magic-users are often a cheat code - bodyguard them fiercely.
67. XP mostly from treasure, not kills.
68. Retreat costs: blocked paths, dwindling resources.
69. Referee is impartial engine of chaos - not plot driver.
70. High-level play: heroic legends, not superheroes. (Think Batman, not Superman)
