Java Thread Synchronization Issues

Consider the FlagCheck class shown below. In a multi-threaded environment, what is the primary issue with this implementation?

public class FlagCheck {
    private boolean flag = false;
    
    public void setTrue() { flag = true; }
    
    public void waitForFlag() {
        while(!flag) {
            // busy-wait
        }
        System.out.println("Flag is true");
    }
}