Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, root@localhost and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Apache/2.2.15 (CentOS) Server at testdomain.com Port 80
Check your apache log and find something like this:
.htaccess: No comments are allowed here
the issue was .htaccess literally not allowing comments after a rule:
deny from 188.8.131.52 #blocked user
Look at the line above. Apache certainly didn't help with its cryptic "no comment allowed here". I wish it would mention what line but it literally meant "no comments allowed at the end of a rule". This is crazy because it worked fine in a previous version of Apache. Another caveat and gotcha of upgrading from an older Apache server that what worked before no longer does.