Mud that is left somewhere in the wellbore when some other fluid is pumped into the well. This can occur when pumping an oil mud into a well to displace a water mud. The bypassed water mud becomes a contaminant in the oil mud when it gets mixed into the circulating system. Drilling mud may be bypassed behind a casing or a liner when pumping cement into the casing or wellbore annular region. This mud-contaminated cement might not set up and might not isolate zones satisfactorily.