Q: I own a bike jointly with my roommate. The bike was lying on the sidewalk and someone tripped over it and was injured. Who is responsible?
A: A stationary hazard, such as this bike, is included in the damage category of bor (pit). Clearly, if one roommate used the bike and left it lying on the sidewalk, he created the hazard and is solely liable, even if the other roommate saw it lying there afterward (C.M. 410:25).
However, if the bike was left standing steadily in its place and fell over, or was knocked over by a passerby, whichever partner knew about it and neglected to take care of the hazard is liable.
Thus, if neither roommate knew about it, neither is liable until the hazard comes to his attention (C.M. 410:22, 26). If one roommate knew about it, he alone is liable. If both knew about the hazard and ignored it, both are liable (Pischei Choshen, Nezikin 7:19-20).