The documentation for ScriptAlias states that "It is safer to avoid placing CGI scripts under the DocumentRoot in order to avoid accidentally revealing their source code if the configuration is ever changed. The ScriptAlias makes this easy by mapping a URL and designating CGI scripts at the same time." httpd 1.x would follow ScriptAliases to correctly call a CGI error handler specified with an ErrorDocument statement, located in a ScriptAliased CGI directory located outside the DocumentRoot as per the above advice. httpd 2.0 does NOT follow ScriptAliases when calling an ErrorDocument, resulting in a 500 Internal Server Error. Nothing is logged to indicate the cause of the error, and the fact that ErrorDocument calls do not follow ScriptAliases is not documented anywhere (that I've found, at least). Since it is not documented that httpd 2.0 does NOT follow ScriptAliases when invoking an ErrorDocument, and it worked in httpd 1.0, there is no reason to suspect when troubleshooting the failure that this is the problem. There is no obvious way to reproduce the failure that yields any information about why it failed. To further confuse diagnosing the problem, if you try to test your error-handler CGI by setting up an alias to load the CGI directly in a browser, instead of in response to an error, the ScriptAlias will be followed, and the CGI will work exactly as you expect it to.
Please help us to refine our list of open and current defects; this is a mass update of old and inactive Bugzilla reports which reflect user error, already resolved defects, and still-existing defects in httpd. As repeatedly announced, the Apache HTTP Server Project has discontinued all development and patch review of the 2.2.x series of releases. The final release 2.2.34 was published in July 2017, and no further evaluation of bug reports or security risks will be considered or published for 2.2.x releases. All reports older than 2.4.x have been updated to status RESOLVED/LATER; no further action is expected unless the report still applies to a current version of httpd. If your report represented a question or confusion about how to use an httpd feature, an unexpected server behavior, problems building or installing httpd, or working with an external component (a third party module, browser etc.) we ask you to start by bringing your question to the User Support and Discussion mailing list, see [https://httpd.apache.org/lists.html#http-users] for details. Include a link to this Bugzilla report for completeness with your question. If your report was clearly a defect in httpd or a feature request, we ask that you retest using a modern httpd release (2.4.33 or later) released in the past year. If it can be reproduced, please reopen this bug and change the Version field above to the httpd version you have reconfirmed with. Your help in identifying defects or enhancements still applicable to the current httpd server software release is greatly appreciated.