The culprit is usually your browser cache. Pronounced "browser cash"
The browser cache is a temporary storage area in your computer's memory or on your hard drive that holds the most recently downloaded web pages. As you jump from web page to web page, caching those pages in memory lets you quickly go back to a page without having to download it from the web again.
Sometimes, old information gets "stuck" in the browser cache and causes problems.
The good news is, it's super easy to fix this by clearing or "refreshing" your cache.
Here's how to do this in the latest version of the four most common browsers:
In Firefox, you can do this by going to Tools -> Clear Recent History and change the time range to "Everything". One thing I like to do (because I have a lot of saved passwords that I don't want to re-enter) is click on the "Details" button and leave a everything unchecked, except for "Cache". (there's also an add-on available that puts a clear cache button on your toolbar)
In Internet Explorer, go to Tools -> Internet Options and under "Browsing History" hit "Delete". This will pop up a list of things you can delete -- the important one here is Temporary Internet Files.
In Google Chrome, click the wrench icon, then choose Options, and click "Under the Hood". Click "Clear Browsing Data" and make sure "Empty the Cache" is selected, and set the "Clear Date" period to "Everything".
In Safari, click the "Edit" menu, and choose "Empty Cache".