There are different reasons this error may occur.

Dev vs Live Site
The most common scenario in which this happens is when the Dev site is being edited,  and then the live site being reviewed. Changes made to a Dev site do not affect the Live or Production site. Confirm you are logged into the Live Site and not the Development or Test site.

Caching
Another scenario in which this happens is due to caching, either within your browser cache or from a caching plugin in the site. 
Browser Caching: Here is a 3rd party link on how to clear cache: http://www.refreshyourcache.com/en/home/. Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.
Plugin Caching:  If your site includes a caching plugin, typically used to speed up the loading of pages in WordPress, it's possible the cached version has cleared itself. This issue will resolve itself when the cache clears itself. If you have access to clear cache for the website, you may perform this manually.
  1. Locate the caching plugin for the site. This can vary between sites. 
  2. Click the clear cache button. The language will vary between plugin types.
Remote Desktop Session
If you are making changes to your WordPress site using different computer accessed over a remote connection this can also cause problems when updating your site.
  1. In some instances the remote desktop session will cache your changes/webpage and it will look like the site has been updated. When users go to the live site they will not be able to see the changes and the files that you uploaded to the site will not be accessible to other administrators or viewable to visitors to of your site. 
  2. If this occurs, log out of your WordPress account and disconnect from the Remote desktop session.
  3. Log into your WordPress site directly by using your web browser to access your site over the active internet connection and login as an administrator.
  4. If you notice that the changes you made to the site and any files you uploaded are missing, the problem is the remote connection.
  5. Make the changes without using the remote connection and they should apply once saved.