Your "proper answer" is what the US tried to do in Iraq, which is now considered the perhaps biggest mistake in US history.
But what I think OP meant, is that the bad things which happened in Iraq were not the intended outcome of what the US did. Those other, more horrible things were intended consequences, not mistakes.
Not quite sure why. Some tries:
1. A mistake implies a single decision.
2. Slavery worked as intended, right? It was more evil than mistaken.
Either way, you sure derailed my derailing of the topic, so I'll stop here :)