If that can help: it occurs when I use the same kind of bitset as in http://lua-users.org/lists/lua-l/2011-08/msg00374.html (which I cannot replace by something else yet for unrelated reasons). Apparently the function that sets a bit to 1 is skipped sometimes: local bit_set_1 = function(_bset,_byte,_bit) _bset[_byte] = bor(bset[_byte],bmask_1[_bit]) end -- Pierre 'catwell' Chapuis