What do you do if your friend's got a bad idea

So a friend calls you up and tells you that he/she's thought of this great idea, going to try it out, and wants to hear your thoughts.
And you think it's a terrible idea. Just absolutely terrible. You've never done it before, but you don't think it's any good. You feel that your friend's just wasting his/her time and could be better off doing something else.

What do you do?
Do you actively try to discourage your friend from going through with it?Do you try to support your friend despite believing it is a terrible idea?
Somewhere in the middle?

Depends.
Is it dangerous or very high risk or a scam? If so, yes I'd firmly advise against it.
Do I just think its lame or stupid or just "risky"? Nope. Let them try their luck, may end up surprised. Might give a little input here and there and try to be supportive and helpful. No reason to be a downer about everything and people will accomplish nothing if not for trying.

If it's dangerous just straight up tell them it's a bad idea. 

Crack out the camera phone and document this fail for future giggles.

Just do what my friend does when I tell her my idea; just shoot it down with logic.

Slap him with a dose of "your ideas suck" 

I say, "DO IT!" Even if it's dangerous because YOLO! Lol. But no. I'd be like, "Sure go ahead. As long as I'm not paying for whatever medical bill that comes out of this, you can do whatever you want."

It depends because there were some ideas that I thought were stupid but turns out that it worked pretty good. And if you watch TV there's a bunch of stunts that would be considered a bad idea to regular people. But because they practiced and know their own limitations and safety precautions they are able to perform and entertain us.

And you know what they say the bigger the risk the bigger the reward.

Basically if your friends' mind is set, they will do it no matter what you say. But if you are a good friend you still need to let your friend know your opinion. It's all about how you say it. Don't rant and rave about how stupid the idea is. Say that you are happy your friend is so excited and then ask some questions about the activity, things that you are sceptical about. If you are still not convinced that it is a good idea then tell your friend that you have some doubt about it and advise the person to be careful. In the end there is no more than that you can do. If you right out shoot down the idea with your opinion, your friend would more than likely go into a defensive mode and not listen to you at all. Better to get them to think about the risks themselves that to try to talk them out of it. And if they don't follow your advise then be a good freind after they are hurt also, and never say i told you so. Hope it works out!

My friends and I are both full of ideas other people would consider terrible.

But I am sure those men who sought to discover the "New World" from Europe were considered idiots, with a terrible idea.

I am sure it was idiotic to think the Earth wasn't the center of the Universe.

And all those other idiots who dared to question conventional norms.

It all comes down to how intense you want to live. If you're interested in living fully, then you will have many stupid ideas, and do many stupid things, I am sure.

Depends on the friend.
Some of my friends will argue with me over pretty much everything until one of us just gets mad and leaves. Those friends, I might say my opinion once, but if they argue, I would probably just tell them to go ahead with the idea unless it's like super dangerous.
For the friends who actually listen to me and take my advice, i'll give them my honest opinion.

I would just say "Hey mannosuke, stop making another one of those bad threads"

This is a tough one. While on one hand I want to be supportive but it's a fine balance like what if they don't take kindly to my critique? There're some who are so set on achieving their goal, nothing not even a Tsunami can break their determination. So I guess it depends.