13.1 In all your probing of the target application, monitor its responses for error messages that may contain useful information about the error's cause, the technologies in use, and the application's internal structure and functionality.
13.2 If you receive any unusual error messages, investigate these using standard search engines. You can use various advanced search features to narrow down your results. For example:
“unable to retrieve” filetype:php
13.3 Review the search results, looking both for any discussion about the error message and for any other websites in which the same message has appeared. Other applications may produce the same message in a more verbose context, enabling you to better understand what kind of conditions give rise to the error. Use the search engine cache to retrieve examples of error messages that no longer appear within the live application.
13.4 Use Google code search to locate any publicly available code that may be responsible for a particular error message. Search for snippets of error messages that may be hard-coded into the application's source code. You can also use various advanced search features to specify the code language and other details, if these are known. For example:
unable\ to\ retrieve lang:php package:mail
13.5 If you receive error messages with stack traces containing the names of library and third-party code components, search for these names on both types of search engine.