404 display issue

Hello,

When I enter a non existing link (example) all I get is lines of html code. I have a 404 page installed in my theme but it doesn’t display properly. By the looks of it there is some kind of problem in the header, but I don’t have this problem of any of my other pages(they all use the same header) so I can’t really see the problem. Is this a common problem related to the 404 page? I have checked my code a million times and can’t see what my next step could be.

Any ideas are much appreciated.