Problem with HashSet

So I'm having a problem with the implementation of HashSet getting stuck in an endless loop working through it's internal implementation which uses a linked list. For some reason the list is getting linked back to itself after a resize operation or a series of removal and resize operations.

Basically it jams up in anywhere the linked list is used. E.g. this becomes an endless loop:

  bool SlotsContainsAt(int index, int hash, T item)
         {
             int current = table[index] - 1;
             while (current != NO_SLOT)
             {
                 Link link = links[current];
                 if (link.HashCode == hash && ((hash == HASH_FLAG && (item == null || null == slots[current])) ? (item == null && null == slots[current]) : comparer.Equals(item, slots[current])))
                     return true;
 
                 current = link.Next;
             }
 
             return false;
         }

So I believe this only happens after a resize operation or a significant number of removals. I can't work out what's wrong though - I've built my own version from the Mono source and added some debugging, here's the original:

    void Resize()
     {
         int newSize = HashPrimeNumbers.ToPrime((table.Length << 1) | 1);

         // allocate new hash table and link slots array
         var newTable = new int[newSize];
         var newLinks = new Link[newSize];

         for (int i = 0; i < table.Length; i++)
         {
             int current = table[i] - 1;
             while (current != NO_SLOT)
             {
                 int hashCode = newLinks[current].HashCode = GetItemHashCode(slots[current]);
                 int index = (hashCode & int.MaxValue) % newSize;
                 newLinks[current].Next = newTable[index] - 1;
                 newTable[index] = current + 1;
                 current = links[current].Next;
             }
         }

         table = newTable;
         links = newLinks;

         // allocate new data slots, copy data
         var newSlots = new T[newSize];
         Array.Copy(slots, 0, newSlots, 0, touched);
         slots = newSlots;

         threshold = (int)(newSize * DEFAULT_LOAD_FACTOR);
     }

And here's my debug version that does show that it is trying to link to itself just before my endless loop happens:

     void Resize()
     {
         int newSize = HashPrimeNumbers.ToPrime((table.Length << 1) | 1);

         // allocate new hash table and link slots array
         var newTable = new int[newSize];
         var newLinks = new Link[newSize];

         for (int i = 0; i < table.Length; i++)
         {
             int current = table[i] - 1;
             while (current != NO_SLOT)
             {
                 int hashCode = links[current].HashCode;
                 int index = (hashCode & int.MaxValue) % newSize;
                 if (newTable[index] != current+1)
                 {
                     newLinks[current].Next = newTable[index] - 1; //NO_SLOT the first time
                 }
                 else
                 {
                     UnityEngine.Debug.LogError("HashSet::Trying to link to self");
                 }
                 newTable[index] = current + 1;
                 current = links[current].Next;
             }
         }

         table = newTable;
         links = newLinks;

         // allocate new data slots, copy data
         var newSlots = new T[newSize];
         Array.Copy(slots, 0, newSlots, 0, touched);
         slots = newSlots;

         threshold = (int)(newSize * DEFAULT_LOAD_FACTOR);
     }