You're right its very hard to overdose on vitD. When you recieve massive amounts of it via sunlight and food your body can regulate it through natural biological and chemical processess.

Where people run into trouble is over doing it on a medical prescription of 50k units. Usually this is prescibed to people who are chronically low on vitD over a period of months or years (this could also be a sign of something else is wrong).

This can be prescribed as something like one pill twice a week. People can mess this up and do 50k units a day by accident. That could lead to kidney problems like stones.