What is the hardest way to learn the lesson that Wikipedia is not reliable?

I can think of a number of scenarios, but probably the roughest would be to be a lawyer and have a judge throw out your entire case because you cited Wikipedia. It appears that lawyers have actually tried to use Wikipedia in court, to little benefit to their client.

So yeah, to learn that lesson in college would certainly be better than to hear it from a judge.

A less likely scenario: You're on a game show going for the million dollars. You call your lifeline person, but he doesn't know the answer. He looks it up on Wikipedia and tells you. It turns out the answer is wrong. Wikipedia actually had it right most of the time, but at the exact moment that it really counted, the page was vandalized with the wrong information. It would be little consolation that the information was corrected minutes after you called on your lifeline.

It's much better to hear it from a teacher in college than out in the real world. In the real world, there are often very real consequences for having the wrong information. In college, you get a few points shaved off your grade.

When your teacher gave you a F on a report that is based on Wikipedia

College. If you use it in a real-life situation, you can look like a fool and possibly jeopardize your career.

Always find a source to verify when you see on that site. Always.

When the doctor explains to you that if you had distrusted the medical information on Wikipedia and consulted him/her earlier, you wouldn't have irreversible physical damage now.

Well, yeah. After all, wouldn't you rather have someone tell you something than learn yourself the hard way?

Consider the scenario where someone tells you that "This snake is poisonous" and the alternative where you're like, "I wonder if this snake is poisonous... lets get it to bite me and see what happens!"

College. When your teacher gives you an F, you'll learn quickly.