dired: it is actually a marked file off the screen you will act on

[Dan Jacobson]

Try this: do dired on a large directory of lots of similarly named
files.  Now mark one of the files with "m", and move it off the screen
with a C-v. Now use ! (dired-do-shell-command) on the file under the
cursor.  Note how easy it is to think that the file under the cursor
is the one to be acted upon!

Therefore some visual extra warning should appear when
dired-do-shell-command will act on a marked file and not the one under
the cursor.

Yeah, we are prompted with the file name, but like I said, suppose
they are very similar.

Same for any dired command that plans to act on marked files.

Some kind of unignorable minibuffer signal.