[SalesForce] What happens to a Scheduled Job if the Apex Flex Queue is full

Building on the approach recommended by @Amit

/**
* @author Scott Covert
* @date 8/3/2015
* @description Salesforce Utility Class
*/
public with sharing class SFUtils {
    /** 
    * @author Scott Covert
    * @date 8/3/2015
    * @description Determines if the current org has the Apex Flex Queue enabled
    * @return Boolean A boolean value representing whether or not the Apex Flex Queue is enabled for the current org
    */
    public static Boolean detectApexFlexQueue() {
        String sfURL = System.URL.getSalesforceBaseURL().getHost();
        PageReference pageRef = new PageReference('https://'+ sfURL +'/apexpages/setup/viewApexFlexQueue.apexp?retURL=%2Fui%2Fsetup%2FSetup%3Fsetupid%3DJobs&setupid=ApexFlexQueue');
        String pageRefBody;
        if (Test.isRunningTest()){
            pageRefBody = '';
        }
        else{
            Blob pageRefBlob = pageRef.getContent();
            pageRefBody = pageRefBlob.toString();
        }
        // Count # of times 'setupHighlightLeaf' appears in body
        Integer highlightLeafMatches = pageRefBody.countMatches('setupHighlightLeaf');
        // In orgs without the Apex Flex Queue enabled there should be no highlighted leafs
        // meaning there should only be one match from the HTMLTreeNode initialization
        if (highlightLeafMatches<=1){
            // Apex Flex Queue is not enabled
            return false;
        }
        else {
            // Apex Flex Queue is enabled
            return true;
        }
    }

}

Its basically the issue with the scheduled job names. When Scheduled Job2 is running and trying to scheduled next set of Scheduled job its finding that a job named Scheduled Job0 is already running or scheduled. So you should develope a logic to name your scheduled jobs to be unique.

You can use the current timestamp in your scheduled job name like this:

System.schedule('Scheduled Job0'+System.now(), '0 0 * * * ?',  new SchedulingService ());   
System.schedule('Scheduled Job1'+System.now(), '0 15 * * * ?', new SchedulingService ());
System.schedule('Scheduled Job2'+System.now(), '0 30 * * * ?', new SchedulingService ());
System.schedule('Scheduled Job3'+System.now(), '0 45 * * * ?', new SchedulingService ());

It should make your scheduled job names unique and conflict-less.