This simpler example segmentation faults on my machine :

let () =
  let x = if "a" = "b" then Obj.magic 0 else String.copy "abc" in
  for i = 0 to 100000 do ignore (ref [1]) done;
  print_endline x

-- 
Daniel de Rauglaudre
http://pauillac.inria.fr/~ddr/