I observe the following unexpected behavior:

--- a.ml ---
try
  … some code ...
with e ->
(*  Printf.eprintf "%s\n" (Printexc.to_string e); *)
  Printexc.print_backtrace Legacy.stderr
------

Compiling as ocamlopt -g a.ml gives:
$ ./a.out
Raised at file "fool.ml", line 162, characters 13-22
Called from file "bar.ml", line 13, characters 5-56
Called from file "bar.ml", line 43, characters 4-63

Now this doesn't say which exception was raised, so I uncomment the
commented line. I'm expecting the identical output, but with the exception
printed first. However, I get:

$ ./a.out
Not_found
Raised at file "hashtbl.ml", line 93, characters 19-28

So it appears that printing an exception changes the backtrace. Is that
possible?

Unfortunately, I cannot reproduce this behavior in a minimal example. The
following script behaves as expected:

--- b.ml ---
let f () = raise (Failure "")
let g () = f ()
let _ =
  try g()
  with e ->
    (* Printf.eprintf "%s\n" (Printexc.to_string e); *)
    Printexc.print_backtrace stderr