The Collection constructors taking a Collection were not thread-safe.
Vector and ArrayList were fixed by

6355660: (coll) Vector(Collection) constructor is not thread safe
6347106: (coll) Make ArrayList(Collection) more threadsafe

but PriorityQueue (and indirectly PriorityBlockingQueue) remain.

The fix is more involved because providing thread-safety without loss of
performance requires switching from 1-based to 0-based arrays internally.
This should be done anyways. There is no reason to waste the initial
element of the internal backing array.