This program

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import java.util.*;
import java.util.concurrent.*;

public class Bug {
    public static void main(String[] args) throws Exception {
Map m = new ConcurrentHashMap();
m.put(1,2);
Iterator it = m.entrySet().iterator();
if (it.hasNext()) {
m.remove(1); // sneaky
System.out.println(it.next());
}
    }
}
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prints

1=null

which is very surprising, since 1 was never mapped to null,
and in fact, it could never be.