Unsafe check of initiated in HybridInstance#init()

    private void init() {
        if (initiated == true)
            return;

        synchronized (this) {
            if (initiated == true)
                return;

            if (realizationEntries == null || realizationEntries.size() == 0)
                throw new IllegalArgumentException();

Suppose there are two threads executing the above code.

Thread1 uses the value read from field initiated in the condition initiated == true. It sees that the condition is false.
Thread2 checks variable initiated, reading it after Thread1 assigns to initiated but before some of the correlated field assignments take place. It sees the condition initiated == true as being true. It continues on before the critical section has completed, and can read data changed by that critical section while it is in an inconsistent state.