I ran into this issue with Ruby 1.9 (not verified w/ 1.8.7) and the SDL C libraries on Mac OSX.  The specific issue here is related to the Objective-C wrapping of SDL (I guess?).  This issue is not necessarily seen on other platforms.

When running Autospec, noise abounds!

/Users/peterfitzgibbons/.rvm/rubies/ruby-1.9.1-p378/bin/ruby /Users/peterfitzgibbons/ruby-projects/wizardry/vendor/gems/rspec-1.3.0/bin/spec --autospec /Users/peterfitzgibbons/ruby-projects/wizardry/spec/gosu_star_catcher_spec.rb -O spec/spec.opts
Profiling enabled.
/Users/peterfitzgibbons/ruby-projects/wizardry
./Users/peterfitzgibbons/ruby-projects/wizardry
./Users/peterfitzgibbons/ruby-projects/wizardry
.

Top 10 slowest examples:
0.2427560 GosuStarCatcher when initialized should have set dimensions
0.1411460 GosuStarCatcher when initialized should have a background image
0.1077080 GosuStarCatcher when drawn should have background

Finished in 0.526593 seconds

3 examples, 0 failures
2010-03-28 21:47:13.642 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x1005200f0 of class NSCFNumber autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.645 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x100567690 of class NSConcreteValue autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.646 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x11236f080 of class NSCFNumber autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.647 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x1005300c0 of class NSConcreteValue autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.649 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x100538b40 of class NSCFDictionary autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.667 ruby[72302:903] *** attempt to pop an unknown autorelease pool (0x100db0a00)
2010-03-28 21:47:13.668 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x101976320 of class NSCFNumber autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.668 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x1019f1d30 of class NSConcreteValue autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.688 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x11236f080 of class NSCFNumber autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.689 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x100589980 of class NSConcreteValue autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.693 ruby[72302:903] *** __NSAutoreleaseNoPool(): Object 0x1019c1710 of class NSCFDictionary autoreleased with no pool in place - just leaking
2010-03-28 21:47:13.706 ruby[72302:903] *** attempt to pop an unknown autorelease pool (0x100db1a00)

Solution?  Run autospec in terminal window-x with a file redirect of stdout :

autospec 2>>autospec.log

If you have issues, tail the log.  Most of the time, your useful info will be written to &1 (stdout) because autospec is trappping all exceptions and wrapping.

Yeay, nice output!

/Users/peterfitzgibbons/.rvm/rubies/ruby-1.9.1-p378/bin/ruby /Users/peterfitzgibbons/ruby-projects/wizardry/vendor/gems/rspec-1.3.0/bin/spec --autospec /Users/peterfitzgibbons/ruby-projects/wizardry/spec/gosu_star_catcher_spec.rb -O spec/spec.opts
Profiling enabled.
/Users/peterfitzgibbons/ruby-projects/wizardry
./Users/peterfitzgibbons/ruby-projects/wizardry
./Users/peterfitzgibbons/ruby-projects/wizardry
.

Top 10 slowest examples:
0.2427560 GosuStarCatcher when initialized should have set dimensions
0.1411460 GosuStarCatcher when initialized should have a background image
0.1077080 GosuStarCatcher when drawn should have background

Finished in 0.526593 seconds

3 examples, 0 failures