Hello,
One of our client was getting "Service Unavailable" error on website. Further checking it was generating below errors on the server.
QuoteINFO: validateJarFile(/home/username/public_html/WEB-INF/lib/selenium-server-standalone-2.52.0.jar) - jar not loaded. See Servlet Spec 2.3, section 9.7.2. Offending class: javax/servlet/Servlet.class
There was compatibility issue of selenium-server-standalone-2.52.0.jar with the Tomcat server as it is a standalone server.
In order to fix issue don't use the standalone selenium jar, instead use the small non-standalone selenium jar and also use all the jars it depends on.
Download the full zip from HERE (http://docs.seleniumhq.org/download/)
After that Open the zip and copy selenium-java-2.53.0.jar in WEB-INF/lib folder and restart the Tomcat service.
That's it. Now your website should work fine :)