Java Thread Memory Visibility

Consider the FlagChecker class shown below. What is a potential issue if the flag instance variable is not declared as volatile?

public class FlagChecker {
    private boolean flag = false;
    
    public void runThread() {
        new Thread(() -> {
            while (!flag) {
                // Busy waiting
            }
            System.out.println("Flag is true, proceeding.");
        }).start();
    }
    
    public void setFlag() {
        flag = true;
    }
    
    public static void main(String[] args) throws InterruptedException {
        FlagChecker fc = new FlagChecker();
        fc.runThread();
        Thread.sleep(100);
        fc.setFlag();
    }
}