Stateful lambdas lead to non-deterministic stream behavior.

NondeterministicStream.java

List<String> ss = ...;
List<String> result = ...;

Stream<String> stream = ss.stream();

stream.map(s -> {
        synchronized (result) {
          if (result.size() < 10) {
            result.add(s);
          }
        }
    })
    .forEach(e -> { });

// The contents of result will depend on the order of execution.
// If this stream is run in parallel, result.add(s) is called
// for some elements, depending on which thread/spliterator is
// called first.