Here is a session I had with irb:

[200]~% irb
irb(main):001:0> require 'digest/sha1'
=> true
irb(main):002:0> Digest::Sha1.new.hexdigest('fdjslk')
SystemStackError: stack level too deep
        from d:/ruby/lib/ruby/1.8/digest.rb:9:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
... 22338 levels...
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_get'
        from d:/ruby/lib/ruby/1.8/digest.rb:18:in `const_missing'
        from (irb):2irb(main):003:0>
irb(main):004:0* exit
[201]~% ruby -v
ruby 1.8.6 (2007-03-13 patchlevel 0) [i386-mswin32]

I tried this from two computers and got the same results.  What am I
doing wrong?

Thanks!

Tim