Very simple explanation for why this happens (not how it happens)......
quitting time on friday is 4 pm,
it is friday,
boss says this tank needs cleaned today,
it takes at least 1 hour to clean it,
it is now 3:30 pm,
you and Bubba spend 20 seconds contemplating which shortcuts you can take to knock a few minutes off,
you and Bubba figure if you keep it sealed it will get hotter quicker (and therefore cleaner quicker),
You and Bubba decide NOT to open any venting valves until you are almost done,
while cleaning you and Bubba begin discussing the plans for the weekend,
Bubba is gonna drink alot of beer,
you are gonna drink alot of beer,
you both agree that you both will drink alot of beer,
Bubba says "Its 3:55! 5 minutes til quitting time",
Both you and Bubba figure the other opened the vents,
Bubba and you get all the stuff put away and make a mad rush to the time clock,
You and Bubba punch out at EXACTLY 4:01 PM,
on the way to the parking lot you both hear metal making sounds it ain't supposed to make,
Neither of you think anything about it,
the boss comes running up screaming "you guys destroyed that tank car"
Bubba says "It ain't us boss!, we was way out here!"
moral of the story-
NEVER EXPECT QUALITY ON A RUSH JOB ON FRIDAYS!