Not the biggest Apache fan here, but today working on a server while setting up Apache as a reverse proxy for the JBoss application, after trying to restart the apache service I faced the following issue Apache Configuration Error AH00558: Could not reliably determine the server's fully qualified domain name.
Luckily, a quick Google helped to find the solution:
Long story short (For Debian / Ubuntu)
sudo nano /etc/apache2/apache2.conf
Add a line containing
ServerName 127.0.0.1 to the end of the file