500 server errors such as:

.htaccess: order not allowed here

.htaccess: Options not allowed here

.htaccess: Option MultiViews not allowed here

in your error.log

check you have:

<Directory />
 Options FollowSymLinks
 AllowOverride AuthConfig FileInfo Indexes Limit Options=All,MultiViews
 Order deny,allow
 Deny from all
 </Directory>

in your httpd.conf or apache2.conf

the reason is “all” doesn’t really mean “all”, so you need to add the “AuthConfig FileInfo Indexes Limit”

—Update 14th Jan 2017—
Addition of IP6 addresses added to .htaccess where server does not have an IP6 address assigned to a network card can give a 500 error.