Struggling With the Truth

So time for a basic Question and answer. 

When someone is going to walk into something stupid, and/or do something they will regret, what do you say? 

The reality of this is I'm realizing how much I see people walking right into something stupid. The problem is most of them are completely oblivious to it. For goodness sakes, I've done it countless times. You just wander into a huge mistake, that can involve anything from: regret, hate, anger, depression, a broken heart... that's not even talking about the physical aspects of this. I have hurt myself countless times doing very stupid things... like breaking and entering...now granted, I did lock the keys in the house that I was breaking and entering in...But that's another story for another time. 

So my question is, more specifically, when should you interfere and when should you stay out of it? And how exactly do you tell someone that they are walking (and they think with both eyes open) into something that they may regret for the rest of their lives? 

How do you bring up taboo subject that you really have no way of telling people?  Especially in the case of something you have no personal experience in? 

It's like a clown trying to tell a computer techie that his computers could be more efficient if [insert crazy/practical/whatever idea here). And it doesn't matter if your idea is actually revolutionary... people will just laugh at you. Or call you a fool. 

There is just nothing wise... or even intelligent about doing something like that. 

So do you wait for someone credible to tell the techie? Do you find someone credible TO tell the techie for you? 

The problem is, some people need the mistakes to grow. Where do you draw the line between someone who needs to be stopped and someone who needs to be taught??

Are you responsible for the mistakes of someone when you know that they will not listen to you?

And yet again, I'm left with more questions then answers. 

Katydid out.