Source? There have been many articles on HN showing the RDA to be ~10x too low (something like 5,000 IU) and that the daily safety limit to be significantly higher than that (something like 30,000 IU).

Edit: for clarity I am not saying it is impossible to overdose on oral tablets, but rather that with most tablets 400 IU to 1000 IU and the safe limit so much higher than these, it seems like it would be extremely unlikely for someone to be taking 30+ tablets daily. Not impossible, but not easy either.

I would say it's almost impossible with typical packaging. What makes it easy?