Programming puzzle

Inspired by the series of programming puzzles from René Stein, I decided
 I'd share any tricky surprises or bugs I came across. Unfortunately, I didn't have to

wait long. I have the following code in the application server's business logic.

public class CisRozpKapService implements ICisRozpKapService {    
    
    private volatile int maxSkupId = 0;
    .
    .
    . 
	
    public int saveSkupina(int kapitola, String nazev) {
        CisRozpKap kap = rozpKapDao.load(kapitola);
        CisRozpKapSkup skup = new CisRozpKapSkup();
        skup.setCisRozpKap(kap);
        skup.setNazev(nazev);
        synchronized(this){
            maxSkupId++;            
        }
        skup.setSkupina(maxSkupId);           
        rozpKapSkupDao.save(skup);        
        return skup.getSkupina().intValue();
    }
    
    .
    .
    .
}

To clarify, maxSkupId has to be unique for each object 
being saved… Can you find and fix the bug?