grep . /; echo $?

[Dan Jacobson]

In grep (GNU grep) 2.5.1, we see
       -d ACTION, --directories=ACTION If an input file is a
              directory, use ACTION to process it. By default, ACTION
              is read, which means that directories are read just as
              if they were ordinary files.

Indeed, the man page should mention that this is the case even with no -d!
Or better yet, exit with a message and a different error value, and
document it.

$ grep . /;echo $?
1
$ strace grep . /
...open("/", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFDIR|0755, st_size=1024, ...}) = 0
read(3, 0x8071000, 32768) = -1 EISDIR (Is a directory)...