Also note that for every Silver ion bound by Thiosulfate/Thiocyanate/whatever a free Halide ion enters the developer. In case of photographic film that means Bromide or Iodide, and both counteract fixer action and development.
Rudeofus - this is interesting. So - is there something to be done to quench or otherwise sequester those pesky free-halide ions? Or is it as simple as ensuring there is sufficient 'excess' developer to mitigate this?