When using tabs for indentation, error messages by the OCaml compiler appear with the wrong alignment:

File "common.ml", line 22, characters 36-55:
22 |            Some (Caml.really_input_string ch (Int32.to_int size))
                                         ^^^^^^^^^^^^^^^^^^^
Error: This expression has type int option
       but an expression was expected of type int
make: *** [Makefile:2: all] Error 1

This ends up becoming annoying if you always use tab characters, as it forces you to go to your text editor and manually go to the columns reported by the compiler to figure out which expression the error applies to.