Explain concurrency error

ExclusiveRunner.java

import java.util.Objects;
import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicBoolean;

/** Executes a runnable as a critical section but without any blocking. 
 *  Number of executions is not guaranteed to match a number of invocations.
 *  Last invocation happens-before last execution.
 * 
 * **/
public final class ExclusiveRunner implements Runnable {
    private final Runnable delegate;
    private final AtomicBoolean requested = new AtomicBoolean(false);
    private final Semaphore semaphore = new Semaphore(1);
    
    public ExclusiveRunner(Runnable delegate) {
        super();
        this.delegate = Objects.requireNonNull(delegate);
    }

    @Override
    public void run() {
        requested.set(true);
        while (requested.get() && semaphore.tryAcquire()) { // Can't tryAcquire() fail spuriously?
            try {
                if (requested.getAndSet(false)) {
                    delegate.run();
                }
            } finally {
                semaphore.release();
            }
        }
    }

}