As the __autoload()-function is triggered by a class_exist()-call you should know that exactly this happens for the SetaPDF_Error class.
To let you define your own error class the script checks if the class already exists and only if it's not available the class will be defined. So the __autoload()-function will be triggered if you didn't define your own error class.
This issue is comes from the old PHP4 days and is
fixed in the SetaPDF core as of version 1.2.2
Here's an example to avoid this problem: