Impossible bug?

gistfile1.md

This error:

Invalid DMA address: 0x5002e0925a00
core/memory.lua:95: DMA address tag check failed

is reported by this function:

local uint64_t = ffi.typeof("uint64_t")
function virtual_to_physical (virt_addr)
   local u64 = ffi.cast(uint64_t, virt_addr)
   if bit.band(u64, 0x500000000000ULL) == 0 then
      print("Invalid DMA address: 0x"..bit.tohex(u64,12))
      error("DMA address tag check failed")
   end
   return bit.bxor(u64, 0x500000000000ULL)
end

... and this happens only in certain configurations of a larger application. (Perfectly unreproducible in certain configurations and certainly reproducible in others.)

How can this happen?