Mayo clinic article concerns excessive supplementation, isolated on vitamin D, specifically.

A hypothesis exists that increasing vitamin K consumption in proportion to the vitamin D, and restricting calcium intake, would eliminate the main symptoms of excessive vitamin D supplementation. Nonhuman animal experimentation confirms, but it has not been tested in humans (likely due to the obvious ethical concerns).

Vitamin D, vitamin K, and calcium are all related.

I have seen guidance specifically for vitamin D2 (ergocalciferol), indicating that adverse symptoms may be observed at lower levels of supplementation than for vitamin D3 (cholecalciferol), but it was quite some time ago, and I don't know if the claim was confirmed or refuted scientifically.

Remember that 60,000 was chosen so that toxicity could be very reliably demonstrated. It does not mean that lower doses are not also toxic.

I'm always worried about how easy it would be for manufacturers to accidentally put more in a pill than the label says.

10,000 IU is only 1.25 mg, and vitamins aren't well regulated. It's not like you'd notice if your supplement contained 100,000 IU.

That happens all the time. Please check https://labdoor.com/ before buying any unregulated drug.

Oh dammit! I wish I had seen this before I went to the drug store...

USP and other certifications are supposed to address this, but the onus is on the consumer to seek out the certification label.

http://www.usp.org/verification-services/verified-mark

Others include NSF, UL, and consumerlab

https://www.consumerreports.org/vitamins-supplements/what-us...

There's still the chance that a manufacturing mistake could result in bad dosage but that applies to all products including medicine.

Vitamin D is particularly worrisome to me because a few milligrams is the difference between therapeutic and toxic.

... and in fact it has happened. With drops for infants that had a 75 times too high concentration:

http://cphpost.dk/news/danish-health-authority-warns-of-toxi...

I know you have faith that taking large doses of vitamin d isn't dangerous, but there is very little evidence supporting its value. We all have to make serious decisions every day without scientific support, so I don't begrudge you the right to do what you believe is best for you. But don't you think it's a little irresponsible to try to convince people that this belief of yours is actually effective? AFICT, there's as good a change that your advice will take ten years off of a life as it has of improving life. And you want to push this on others?