Mishnah 11

If one makes his succah like a hut, or he leaned against a wall, in either case R'Eliezer rules that it is invalid because it has no roof, but the Sages rule that it is valid.

The law of a large reed mat:

If one made it for reclining purposes, it is susceptible to tumah and we may not cover a succah with it. But if he made it for covering a succah, we may indeed cover a succah with it, and it is not susceptible to tumah.

But R'Eliezer says: Whether the mat is small or large, if he made it for reclining, it is susceptible to tumah, and we may not cover a succah with it; but if he made it for covering a succah, we may indeed cover with it, and it is not susceptible to tumah.