On :דף ג, the גמ' says that an אסומ', that a person puts into רשות הרבים, isתחלת עשייתן לנזק . רשי explains that this is so because when it became a בור, (when he put it in רשות הרבים) it was able to do נזק.
But, on :דף ו the גמ' says that a tree or wall that was in a persons private property that fall into the middle of רשות הרבים, and the person doesn’t get rid of it and it does damage, he's חייב. The גמ' continues to say that this is not a case of בור, rather it's learned out from both שור and בור. The reason this is says the גמ', is because its not תחלת עשייתן לנזק. רשי explains that the reason behind this is because when the wall was first built, and the tree first planted they were not able to do damage as a בור. But according to the first גמ' it should be תחלת עשייתן לנזק, because when they first were in רשות הרבים they were able to do damage as a בור?? So what is תחלת עשייתן לנזק? Is it that when it was first in רשות הרבים it was able to do damage as a בור, or is it that when it was first created/recognizable as a wall, tree, rock etc., it was able to do damage as a בור???