Browsers try and remember websites and their contents in a 'cache' to save having to go and get them from the internet when you visit a site again. Sometimes this means your browser has an old version of a web page, or some of its contents in its cache which every so often can cause it to get confused.
If this happens you can force your browser go and get everything fresh from the internet. Depending on your browser there are a different ways of doing this:
On Google Chrome
Windows - Hold the Ctrl key and click the Reload button on the navigation toolbar.
Mac OS - Hold the ⇧ Shift key and click the Reload button on the navigation toolbar.
On Internet Explorer
Hold the Ctrl key and click the Refresh button on the toolbar.
Hold down the ⇧ Shift key and click the Reload toolbar button.