The Rambam says in his introduction to the Mishneh that there is never any argument about things received by tradition, for example that a sukkah is 7 by 7:
ושלא יהיה בחללה פחות משבעה טפחים אורך על שבעה טפחים רוחב
However the gemora (sukkah 7) has three opinions about the size of a sukkah (7 by 7 handbreadths, 6 by 6, or 4 amos by 4 amos), so how can the Rambam claim that?