Comment 1 for bug 565116

If I understand correctly, you keep a reference to an element or part of it. This is expected to break with MCT; from documentation:

    Disadvantages:
        * can move data in memory, i.e. invalidate pointers and references;

References/pointers are invalidated exactly when iterators are invalidated (since the latter are just glorified pointers). In other words, during _any_ insert or rehash operation.