I have the same issue that is described under http://rubyforge.org/pipermail/mocha-developer/2007-August/000423.html.

Code:

  require 'rubygems'
  require 'mocha'

  some_time = Time.at(0)
  Time.expects(:now).returns(some_time)
  puts Time.now

Running it caused:

  /usr/local/lib/ruby/gems/1.8/gems/mocha-0.5.4/lib/mocha/object.rb:40:
  in `expects': undefined method `stub' for nil:NilClass (NoMethodError)
	from test8.rb:5

My gem version:

$ gem list mocha

*** LOCAL GEMS ***

mocha (0.5.5)

Date: 2007-11-29 10:56
Sender: James Mead

I've amended (in revision 198) the misc.rb example as you suggest
to avoid any further confusion. Please close this bug report
if you are happy with this fix. Many thanks.

Date: 2007-11-28 00:10
Sender: Radosław Bułat

I thought that mocha works also outside test methods, but I
see was wrong. My advice is to change examples, because not
all show usage with in framework test context (for example:
http://mocha.rubyforge.org/examples/misc.html). My intention
was to follow with them in irb. Now all is clear for me.
Thanks for response!

Date: 2007-11-27 22:40
Sender: James Mead

Did you see my responses at [1] and particularly [2]? Please
let me know if these don't answer your question. Thanks for your
interest in Mocha.

Cheers, James.
http://blog.floehopper.org

[1] http://rubyforge.org/pipermail/mocha-developer/2007-August/00
0424.html
[2] http://rubyforge.org/pipermail/mocha-developer/2007-August/00
0426.html