In order to test that some code handles an invalid UTF-8 sequence properly, I need to insert an invalid UTF-8 sequence into an existing text file. However, my attempts to do so result in a valid UTF-8 sequence, not an invalid one.

The byte 0xff is always invalid in UTF-8, so I tried to insert it, following the instructions on the ["Inserting Text"](http://www.gnu.org/software/emacs/manual/html_node/emacs/Inserting-Text.html) page of the Emacs manual:

```
c-q 377 <RET>

```

A funny-looking character got inserted into the file:

```
-OP INC       ÿ3713 SOUTHS
              ^

```

However, when I examine the contents of the file using the Unix command `od -c <path>`:

```
0001240   C  \t 303 277   3   7   1   3       S   O   U   T   H   S   H
                ^^^ ^^^

```

I can see that a valid UTF-8 sequence 303 277 (octal) has been inserted, not the single byte 377 that I tried to insert.

How can I insert an invalid UTF-8 sequence into a file?