"And he not cover it" — the Torah addresses liability for an uncovered pit. The Mekhilta adds a crucial qualifier: "and he not cover it properly." This distinction between proper and improper covering generated a series of practical rulings.
If a person covered a pit properly — with a covering strong enough to prevent accidents — he is exempt from liability. If the covering was improper — too flimsy to actually protect passersby — he remains liable as if the pit were uncovered.
The Mekhilta then explores more complex scenarios. If two partners own a pit jointly and one of them covers it, but then the other partner comes along and uncovers it, the second partner bears full liability. He is the one who created the danger by removing the covering.
What if partners covered the pit together, but one of them knew it had become uncovered while the other did not? The one who knew is liable. The one who was unaware is exempt. Knowledge transforms legal status. The pit itself is the same whether you know about it or not, but your liability depends entirely on your awareness.
These rulings constructed an entire law of negligence from a few words of Torah. The key factors are: adequacy of precautions, identity of the person who created the danger, and state of knowledge. The Mekhilta built a sophisticated framework for assigning responsibility in multi-party situations, all derived from the seemingly simple phrase "and he not cover it."