I assume you mean in HTML.
Basically you don't. Ususally it means the page has completed loading, but something returned a fail. This can be because your security settings are high, and your computer didn't allow the server to give you some part of the page it wanted to (which, while annoying, is probably a good thing). It can also mean that you are using a browser (or version of browser) that the site wasn't built for (this is particularly true for script and multimedia type things rather than HTML problems themselves (because HTML itself merely ignores things it doesn't understand).
Certainly the only times I've seen such errors is when a website wants to do things which my security settings disagree with.....