The case of a courtyard inside a courtyard - what happens if the inner people established an eruv, but the outer people did not? Or the reverse? Or if they made an eruv for each separate courtyard, but not together? And all the other permutations. Note that the principle here precludes individuals from establishing eruvin. Also, R. Yosef on the case of inner/outer courtyards - in which he incorrectly states the halakhah, and the person who had taught the halakhah. R. Yosef's illness on display - and he explains his own mistake.