You have exceeded the maximum number of scheduled Apex jobs (100)

The 100-job cap counts every Apex class scheduled with System.schedule(...). If your org has been running for years, old jobs accumulate — schedules created during pilots, testing, abandoned features.

Audit existing scheduled jobs

SELECT Id, CronJobDetail.Name, NextFireTime, State
FROM CronTrigger
WHERE CronJobDetail.JobType = '7'   -- Scheduled Apex
ORDER BY CronJobDetail.Name

(JobType = '7' is Scheduled Apex; other JobTypes are Batch, Future, etc.)

The result shows your job count. Over 90 = warning territory.

Abort stale jobs

List<CronTrigger> jobs = [
    SELECT Id, CronJobDetail.Name FROM CronTrigger
    WHERE CronJobDetail.JobType = '7'
];
for (CronTrigger ct : jobs) {
    if (ct.CronJobDetail.Name.startsWith('Old_')) {
        System.abortJob(ct.Id);
    }
}

System.abortJob removes a job immediately. Other jobs are unaffected. Once aborted, the slot frees and you can schedule a new one.

Fix: consolidate schedules

If you have 50 hourly jobs, consider one master scheduler that calls all of them:

public class MasterScheduler implements Schedulable {
    public void execute(SchedulableContext ctx) {
        Database.executeBatch(new JobA());
        System.enqueueJob(new JobB());
        new JobC().run();
    }
}

One job in the queue, multiple jobs execute. Cleaner, easier to monitor, doesn't bump against the 100 cap.

Fix: use Queueable chains for one-time deferred work

System.schedule(...) is for recurring schedules. For "run X in 5 minutes" you don't need a schedule:

System.scheduleBatch(new MyBatch(), 'OneTimeRun_' + System.now().getTime(), 5);

Or use enqueueJob with a delay flag. These don't accumulate persistent CronTrigger rows.

A subtle source: scheduled tests in test classes

@isTest static void testScheduling() {
    System.schedule('Test_' + DateTime.now().getTime(), '0 0 1 * * ?', new MyJob());
    Test.startTest();
    Test.stopTest();
}

Tests sometimes fire System.schedule and forget to abort in the teardown. Each test run accumulates one more CronTrigger row. After enough test runs, you blow the cap.

Fix: always wrap in Test.startTest() / Test.stopTest(). The platform automatically aborts test-context schedules when the test ends.

When the org has too many job runs in flight

Different cap, similar feel. The org allows 5 concurrently running scheduled jobs. If five are simultaneously executing, the next scheduled fire-time is queued. Long-running jobs blocking new ones can manifest as "schedule limit exceeded" though that's misnamed — it's actually the in-flight cap.

Keep individual scheduled jobs short; offload heavy work to Batch Apex called from the schedulable.