Sojourning Under the Sun: Sabbath Stall (2) (Numbers 20)

What is so rebellious about Moses striking the rock two times?  We might think that this is a question that is out of line, but Moses is commanded to strike the rock in Exodus 17.  Why is it okay for him to strike the rock in Exodus 17, but not okay for him to strike the rock in Numbers 20?  What does Moses do that is so bad?  Is God just a moody being who judges people in a very vindictive and unpredictable way?  What did God intend the people to see through Moses?