>   If I open a file in emacs, enable both hexl-mode and
 > auto-revert-mode in its buffer and then modify the content of the file
 > externally, the buffer gets refreshed but its content seems... rather
 > littered. C-l doesn't fix it.
 >
 >   I have similar problems with "auto-revert-tail-mode", but not when I
 > revert the buffer "manually" with M-x revert-buffer.
 >
 >   It seems that when the buffer gets refreshed, it behaves as if
 > hexl-mode was not here (for instance with auto-revert-tail-mode, it
 > just appends the text, without printing the hex values) but I'm not
 > sure about what's going on here.

It's because `auto-revert-mode' doesn't reset the major mode when
reverting the buffer and `hexl-mode' does nothing when the buffer is
already in `hexl-mode'.  Does the attached patch give good results?