The fact that you cannot foresee all consequences is not a fundamental error of consequentialism, any more than not knowing all applicable virtues and which to prioritize is a fundamental problem or not knowing exactly what God wanted in every specific instance is a fundamental problem. It's a complication. All moral systems have to deal with human fallibility, and the lack of omniscience is one fallibility that consequentialism has. I've met some very, very good people, but none who were absolutely always acting morally. I'm not interested in arguments for ethical systems that require perfection, since everybody fails in that case.
Let's consider a situation in which somebody else will be badly hurt unless you lie. A consequentialist will weigh the harm done in each case. Somebody who believes in the virtues of telling the truth and helping others will have to make a choice of virtues. A deontologist will have to decide what God requires in that instance. A sanctimonious asshole will try to remain morally pure, disregarding the consequences to others. In this case, we see that the consequentialist has more philosophical support for ambiguous situations than a virtue ethicist or deontologist.
I am overall a consequentialist, but I am one who is fully aware of how people work. One of the things you can rely on is people denying responsibility for their choices as much as possible, and consequentialism leaves a big one called "But I meant well!" even when the only person they meant well towards was themselves.
This could be simply avoided by requiring the ends must justify the means, and not the ends you intended but the ends you actually reached.
The test with consequentialist systems, therefore, is how it places your obligations should you discover that a choice you made was not the right one? Are you obligated to act to minimize harm? Are you not morally responsible simply because you had good intentions?
Oh, and how do we define what is a good consequence and to whom? Can I trust you to care about my own opinions when you say you're acting in my benefit?
Or are you going to just be another in a long line of people who mean well but did great wrongs in pursuit of an unattainable good end?
Ultimately, it's not that the ends justify the means, but that they must ultimately do so. This is a lot easier when you don't have too much needing justifying...