Once an idea enters your mind it's hard to get rid of it. Even after you've been proven wrong and know that you are, your brain is wired to stick with the original information. It even influences you subconsciously. This makes it exceptionally difficult to actually feel wrong even when you know you are.
Scientific American has the explanation of this phenomena:
Psychologists asked college students to read an account of an accident involving a busload of elderly passengers. The students were then told that, actually, those on the bus were not elderly. For some students, the information ended there. Others were told the bus had in fact been transporting a college hockey team. And still others were warned about what psychologists call the continued influence of misinformation-that people tend to have a hard time ignoring what they first heard, even if they know it is wrong-and that they should be extra vigilant about getting the story straight.
So how do you get the story straight? The study found that when the college students were warned about the continued influence of information they tended to make fewer inferences based on the old, incorrect "facts" they were given. This didn't solve the problem entirely, as there were still often lingering elements of the original, but it did make an impact.
While reading incorrect information - whether deliberate or not - is unavoidable at times, this study suggests that you're best bet is to simply remember that. If you keep in mind that there's a possibility that what you're reading may contain errors, you're more likely to accept a correction when its made rather than going on believing a lie and spreading misinformation.
Lingering Lies [Scientific American]