Sometimes users see a message similar to:-

Warning: Invalid argument supplied for foreach()

This is not an error message, simply a notice. It means that the variable referenced does not have a value. This is not usually a problem, if the extension is working normally apart from this then this is nothing to worry about.

The reason that you are seeing it is because the level of error reporting is turned up too high for your site. You can turn it down in your site global configuration. It is a good idea to do this anyway on a live site, because attackers can use deliberately generated messages to find information about your filesystem.

If you see this message and no other output it generally means that you have set the wrong parameters for your extension, and there is no output that corresponds to these parameters. For example, if you are using the featured items module and you have specified a category that does not exist the module will not be able to find any joomla articles to output. You need to check your category ids and other module parameters that select the output.