Two errors when javascript calls flash methods

There are two frequent errors I met when I writing javascript and flash communication codes.

1. Error calling method on NPObject!
This error usually means Flash side exception, it might be caused by Flash itself error handling logic, or caused by wrong parameters passed to Flash from Javascript.

2. Uncaught TypeError: Object #<HTMLObjectElement> has no method 'xxx'
This error usually means flash object is not available or no such a method, it might be caused by flash object is not fully loaded, or not found the correct object, or call the wrong flash method (from Flash ExternalCallInterface).