After you have enabled the mambot or install the module, in some environments, you will see the PHP warning messages displayed on the website.
This relates to the PHP error reporting level settings. There are two ways to resolve this:
1. The first method is to update the PHP configuration file php.ini, generally stored at the root level of your web site. If you don't see the file, please create one. Locate the lines which relate to error handling (search for 'Error handling'. The following configuration will allow you to hide warning messages:
display_errors = Off
2. Including following PHP code in the module to turn off the reporting:
First method is preferred, because the setting will apply to the whole web site, while second method requires you to turn off warning in every php code.
To read more, check the following link: http://tutorials.ausweb.com.au/web/Tutorials/Joomla-FAQ/Securing-your-Joomla-Website/