Agree with Taff.
Didn't quite make the RRF Wiki's myth list, but widely believed.
RRF Wiki's myth list
1. You must let a player on the ground up. [laws]14.2 or 15.7 Forbidden practices[/laws]
2. Can't take the second penalty quickly. [laws]21.7 What the opposing team must do at a penalty kick (d)[/laws]. + 21.8 where Free Kicks are concerned
3. Player on their feet in a ruck can play the ball with their hands. [laws]law 16.4 Other ruck offenses (b)[/laws]
4. The referee must stop the game for injuries. [laws]Law 5.4 (a) Injury[/laws]
5. The ball is out when the player has their hands on it. [laws]Law 16.5 offside at the ruck.[/laws]
6. A Kick-off or restart must travel 10m in the air. [laws]Law 13.6 Kick-off of under 10 metres but played by an opponent[/laws]
7. You can ruck a player providing the studs are going backwards. [laws]Law 16.3(f) rucking[/laws]
Law references are my suggestions, feel free to correct them if you think I am mistaken.
8. Number 8 must not pick the ball from the base of an uncontested scrum. [laws]Law 3.6 Uncontested scrums[/laws] doesn't disallow it, so it is presumably allowed.
Other myths which might make the top 10 :
9. No offside in-goal. [laws] Law 22.16 infringements in-goal[/laws]
10. Your own favourite here...