Having been around shops, mechanics, factory-mechs, factory-trainers, and reps over 50+ years of being involved in the love and lore of cars, there's been many instances where the subject of "lower build quality on Monday or Friday" has come up. Some scoff at it, but some others have verified it, from personal experience with odd instances of sloppiness, total indifference, or even practical-joking/sabotage being encountered.
I thought to look up a 2001 calendar, to see what day my X5 was "born"...it was on Friday, the 13th (oh-oh!). Possibly a bad omen, but my X5 has made it 20 years, so maybe not. At least it wasn't on a Monday, which is worse.
I found this forum:
https://skeptics.stackexchange.com/q...-lower-quality, and saw that Sweden has a word for "defective products/cars made on a Monday" (måndagsexemplar)...I think that if the Swedes have a word for the phenomenon, then it truly exists. But, would a German car company (one known for precision) acknowledge it? I think not.
German cars (and various other products: i.e. the Tiger tanks of WW2, for example), are over-complicated and failure-prone for that reason, and even X5's have their failures (though I'm a newbie in X5-land, I've worked on other German..and Swedish...cars that needed nearly-impossible repairs that could've been avoided with simpler engineering). And, though American cars were simpler back in the day, they've been over-complicated, over-engineered, in much the same way nowadays. If there's a gremlin of "bad production days" mixed-in, then failure is assured, at some point.
Though my Friday-the-13th-born X5 hasn't had a "coke bottle been found hanging inside a door panel" (a true occurrence on one or more Detroit assembly lines; though the worst I've actually seen was a few shop rags inside a "new" door), perhaps something else hasn't surfaced yet.