eruvin 12

when a breach is not a breach

when water penetrates a courtyard wall it doesn't always mean it is breached

eruvin 10

can still function with some things missing but not when the gaps are larger than what is standing

R. Yosef forgot the laws he once taught about side-posts and entrances to alleyways.
and
and a discussion about the ratio of the breach in the wall and the standing parts, if they make a complete partition or not.

forgetting and structures.