When I got tired of visually checking md5sum output against what the
download site said it should be, I discovered the -c option.  Unfortunately,
the web site for downloading the new Mandriva distribution only showed the
sum on the web page, there was no file to download.  OK, easy, right?  I
highlighted the line, pasted it into a text editor, saved it as
mandrivamd5.txt and ran md5sum -c mandrivamd5.txt.  I got the report
claiming the file contained no valid line.  Turns out md5sum wants TWO
spaces between the checksum value and the file name.  Of course, while the
may have been 2 spaces originally, it would appear the friendly reformatting
of html (or whatever) decided 2 spaces were redundant and removed one.  So
be forewarned ...

On the other hand, the man page on md5sum seems to indicate that the
checksum value and the filename should be separated only by an asterisk (for
a binary) or a space (for text).  Hmm, that seems to put the man page at
odds with the behavior of the md5sum program.  How curious!

Another problem with most sites with a link to an MD5SUMS file, usually when
I would click on the link it would simply display the file rather than
downloading so I could use it.  Then I discovered I could right click and
choose "Save link as ...".  Fairly counterintuitive IMHO.  Makes me wonder
why two links on similar lines (typical ftp directory style page) are
treated differently by the browser.  Yes, I am sure it has to do with the
file type, but I don't really understand why it should.

-- 
The nice part about being a pessimist is that you are constantly being
either proven right or pleasantly surprised.
  - George F. Will