Got the "can't activate" Gem::Exception Blues? gem cleanup!

What to do when rubygems can’t activate a version of a gem because it’s already activated a different version of the same gem? My Google magic wasn’t good enough to find this one quickly, so I’ll happily point out Jesse Hu’s nearly one-year-old post about the same problem, descended from a Ruby on Rails thread on ruby-forum.com where, unsurprisingly, it’s Jeremy McAnally who knows that gem cleanup is the way to go.

Took nearly 45 minutes. Freed untold amounts of disk. Much happiness.