euler:rubinius brian$ bin/mspec spec/ruby/1.8/core/array/append_spec.rb --spec-debug -S correctly
..[Debugger activated]

./mspec/runner/actions/debug.rb:5 [IP:18]

rbx:debug> l
   Source lines [1-15] in ./mspec/runner/actions/debug.rb:
      1: require 'mspec/runner/actions/filter'
      2:
      3: class DebugAction < ActionFilter
      4:   def before(state)
  =>  5:     Kernel.debugger if self === state.description
      6:   end
      7:
      8:   def register
      9:     MSpec.register :before, self
     10:   end
     11:
     12:   def unregister
     13:     MSpec.unregister :before, self
     14:   end
     15: end

rbx:debug> n
Stepping out to caller (__block__:34) [IP:23]
[Debugger activated]

./mspec/runner/mspec.rb:34 [IP:23]

rbx:debug> n
Stepping out to caller (each:573) [IP:25]
VM Assertion: cpu_channel_has_readers_p(state, dbg) (cpu_instructions.c:1451)

Ruby backtrace:
 0x14792d8 Array#each+26 in kernel/core/array.rb:574
 0x147926c IO#puts+40 in kernel/core/io.rb:350
 0x1479204 Kernel#puts+28 in kernel/core/kernel.rb:177
 0x14759a4 #+47 in /Users/brian/devel/rubinius/lib/debugger/debugger.rb:159
 0x1474eec Array#each+25 in kernel/core/array.rb:573
 0x1474e80 Debugger#process_command+22 in /Users/brian/devel/rubinius/lib/debugger/debugger.rb:155
 0x14745dc Debugger#activate_debugger+134 in /Users/brian/devel/rubinius/lib/debugger/debugger.rb:141
 0x147456c #+29 in /Users/brian/devel/rubinius/lib/debugger/debugger.rb:57
 0x1474504 Proc#call+22 in kernel/core/proc.rb:84
 0x1474498 Breakpoint#call_handler+25 in kernel/core/breakpoint.rb:106
 0x14741b0 BreakpointTracker#process+86 in kernel/core/breakpoint.rb:209
 0x140606c #+18 in /Users/brian/devel/rubinius/lib/debugger/debugger.rb:34
 0x140574c #+22 in kernel/core/thread.rb:60