I was at a Ford truck plant and saw a truck with several different color panels. I asked about it and was told the truck would be repainted one color and then sold.
I'm sorry but I'll have to take disagreement with this; having actually worked on assembly lines and being the 3rd generation to work at Chrysler (both male grandparents BTW). And although I haven't worked in a Ford assembly plant, their standards aren't going to be vastly different; with some authority, having worked in the Ford engineering laboratories.
For one thing if this was a completed truck, they aren't going to pay someone to disassemble it so it can be run through a 350 degree oven. Yes, they do "spot refinishing", but this is one or two panels, and never over a different color. From a labor hours standpoint, it just isn't worth it and the complete-repaint materials required don't even exist in an assembly plant. The outside of a factory paint oven is usually littered with complete bodies that will be scrapped because of a paint defect. If they are caught before assembly, it's cheaper to eat the sheetmetal than the labor to repair the defect.
Also consider that all of the sheet metal parts of a vehicle are run through the paint ovens at the same time, on "jigs" for pieces that cannot yet be bolted together (like the cab and bed for a truck). OEM auto paint doesn't contain much by the way of solvents; they rely on heat vs. evaporation or chemical catalysts to cure. To assemble a truck with different color panels would require a dedicated effort to do so.
Now the one scenario "might" occur would be if this was a vehicle destined for a manufacturer's destructive testing. Such vehicles are technically "sold" from the manufacturing division to the R&D division. Factories like these orders because it's a chance to use up unusable, cosmetically defective parts and get "paid" for a completed vehicle. If the truck was still in a body-in-white stage, they might even run it through paint again to get sprayed one-color. There is also a good chance they were just messing with you for laughs. Some dumbasses seem to get great joy in telling outsiders wild stories, even if they aren't true.
I'll also offer that there are so many ways for broadcast sheets to get mixed up that I couldn't even type them all. In the aforementioned black E-body scenario, I must mention that Chrysler upholstered their own seats back then, so they didn't come from a supplier, they came from the Cut & Sew Trim department. Within that department, their would be a car-broadcast printer. It would kick out a sheet that told everything about a specific car, but the only relevant point for that department was a demand of a black interior of XX design. Contrary to the earlier mention cars being trashcans for the disposal of broadcast sheets, if you've ever had to fish one out of the springs, you'll realize it takes some effort to remove it. That's because it wasn't tossed their casually, it signified completion of that demand for specific black seats.
In that era of 5-6 different interior colors and 2-3 different trim levels, there simply wasn't room to inventory 10-18 different seats parallel to the assembly line. So the seats arrive in sequence to the cars coming down the line. A hi-lo driver might have room to stock 7-8 seats in dunnage perpendicular to the seat install area. That will last him about 5-6 minutes. He has to know what cars are coming down the line in what order and stock the area accordingly. But he isn't climbing on/off the hi-lo to read broadcast sheets, he's bringing them from a larger area where they have (usually) already been sequenced. I say "usually" because mistakes happen, (although less often that you'd think). In such a case, if they needed black seats, but blue showed up, someone would haul-*** to grab a set of black seats... broadcast sheet be damned, and by this point it doesn't matter. If you ordered a Big Mac through a drive through, but you received the Big Mac intended for a counter customer, would you know? The repair example isn’t wrong, just one of dozens of possible reasons for broadcast sheets to be mixed. The fact that most of us have the correct sheet shows that mix-ups were relatively rare and unimportant when then they did occur.
It's too bad that people are so quick to disparage the miracle of modern mass production. It's actually quite a complex and amazing study in logistics. It gave us the demand for modern computing power. Who else but manufacturing could afford to purchase such advanced computer systems at scale in the 50s and 60s? If you ever need to win a war, you'll be begging for people with that knowledge.
) but even the OCD anal part of me is starting to worry about you guys. 
