Waste heat boiler for a large incinerator was coming out of an unplanned outage. I get called in at 6 p.m. because of a low steam drum pressure alarm, to find that one of the mercury pressure switches had been stepped on and bent, preventing the mercury from closing the switch. Get it repaired and make the 20 min. drive back home. Get called back in at 8:30 p.m. because "the boiler has no alarms but is not making any steam pressure". Find root valve for PT closed and said a prayer of thanks that the boiler was not yet up to sufficient pressure to start lifting relief valves. Make the 20 min. drive home. Get called back in at 2:00 a.m. becuase "your stupid control valve is not maintaining steam flow setpoint". Climb up on the mezzanine above the steam drum, only to find "my stupid 12 in. control valve" stroking madly, trying to maintain flow. It was at this point I noticed both of the butterfly valves on the bypass line (around the control valve) wide open. Of course, no one had any knowledge of who stepped on, closed or opened any valves...