When the Torah warns that someone who digs a bor – a hole in the public domain – will be held responsible for any damage that takes place if an animal falls in, it includes a caveat. This rule is true only if the person did not cover the bor (see Shemot 21:33-34). Our Mishna rules that if the person did, in fact, cover the bor properly then he will not be held liable for such damages.
The question raised by the Gemara is how could an animal fall into a bor if it was properly covered up? Rabbi Yitzhak bar bar Hanna suggests that it could be talking about a case where the cover became rotten from the inside where it could not be seen, so the person who covered the bor had no way of knowing that the cover would not support the weight of an animal that walked on it.
The approach of most of the commentaries in explaining this case is that the wooden cover was attacked by insects whose entrance holes in the wood are almost imperceptible and the cover may look fine but turn out to be hollow. In his commentary to the Mishna, the Bartenura suggests that the water and moisture of the pit may lead the wooden cover to become rotten on its bottom side that cannot be seen from the top.
The Gemara also raises questions about what sort of animals must the cover be able to support – camels or oxen? At the time of the Mishna, local oxen were relatively small with adult weighing 400-500 kilograms. Full grown camels were much larger, and they often carried loads of up to 250kg – for a gross weight of over 1000kg. Thus, the standard cover that would support an ox may not have been strong enough for a camel.