I hope I'm posting it to the correct newsgroup. I wanted to mail this directly to the developer of boost::intrusive (Ion Gazta?aga), but I can't find his e-mail anywhere, or any way to directly report the bug. I am not a boost developer myself. Position of a contained element modulo m (new_buckets_len) is known to remain equal to its previous position modulo n (old_buckets_len) ONLY IF m < n AND n %m == 0, but the latter condition isn't being checked. The code assumes it can skip rehashing of m first buckets if m < n: //If we are shrinking the bucket array, just rehash the last nodes if(same_buffer && (old_buckets_len > new_buckets_len)){ n = new_buckets_len; } //Iterate through nodes for(; n < old_buckets_len; ++n){ ... } Also, the following comment: //If this is a buffer expansion don't move if it's not necessary if(same_buffer && new_n == n){ ++before_i; ... Needs to be changed simply to "don't move if it's not necessary" - it needs to be checked/may happen when shrinking the hash table also. Here's a suggested bugfix: - check if (old_bucket_len > new_buckets_len && 0 == old_bucket_len % new_buckets_len). - if true, i'th bucket needs to store the contents of all previous buckets at positions i + j*new_buckets_len for j in [0, old_buckets_len / new_buckets_len) spliced one after another. There is no need to check on a per-element basis. I believe this optimization should be done even when the rehash isn't in-place. A special case where this optimization is useful is when using power-of-2 num_buckets. - if false, buckets have to be re-scanned, using the existing code in the body of the existing loop, but starting from n = 0 always: for(; n < old_buckets_len; ++n){ ... } Please correct me if I'm wrong on this.. but just to be sure, I made a test program that verified rehash() can break the container's invariants when shrinking the container in place, which was indeed found to be the case. As a totally unrelated note, I don't understand why iunordered_set doesn't support a default constructor (so that buckets = 0 and num_buckets = 0, given the existence of a rehash() function. I see it like the reset() function in a scoped_ptr. If a "reset" style function is supported, the class shouldn't force you to initialize it at construction time. Then again, it's a matter of taste in design more than anything...