This issue arises even in completely single-threaded programs that have hash tables. Suppose you're iterating over a hash, hitting a callback function for each element which has access to the hash and inserts new elements. During the callback, new elements could be inserted which trigger a growth. If you allow the hash table reorganization, then it has to be reconciled with the in-progress traversal, so that it doesn't miss some elements, or visit some elements more than once.
I think if you insert an element during iteration, it is unpredictable whether said element will be visited during the iteration or not, so it is probably a bad idea even if you wont have a resize.
