あ、再現できた気がします。一番最後のcleanupでmongo-1.7.1がuninstallできないのが問題という事ですね。
% /tmp/local/bin/gem install mongo
Fetching: i18n-0.6.1.gem (100%)
Successfully installed i18n-0.6.1
Fetching: multi_json-1.4.0.gem (100%)
Successfully installed multi_json-1.4.0
Fetching: activesupport-3.2.9.gem (100%)
Successfully installed activesupport-3.2.9
Fetching: bson-1.8.0.gem (100%)
Successfully installed bson-1.8.0
Fetching: mongo-1.8.0.gem (100%)
Successfully installed mongo-1.8.0
Parsing documentation for i18n-0.6.1
Installing ri documentation for i18n-0.6.1
Parsing documentation for multi_json-1.4.0
Installing ri documentation for multi_json-1.4.0
Parsing documentation for activesupport-3.2.9
Installing ri documentation for activesupport-3.2.9
Parsing documentation for bson-1.8.0
Installing ri documentation for bson-1.8.0
Parsing documentation for mongo-1.8.0
Installing ri documentation for mongo-1.8.0
Done installing documentation for i18n, multi_json, activesupport, bson, mongo (27 sec).
5 gems installed
% /tmp/local/bin/gem install mongo -v 1.7.1
Fetching: bson-1.7.1.gem (100%)
Successfully installed bson-1.7.1
Fetching: mongo-1.7.1.gem (100%)
Successfully installed mongo-1.7.1
Parsing documentation for bson-1.7.1
Installing ri documentation for bson-1.7.1
Parsing documentation for mongo-1.7.1
Installing ri documentation for mongo-1.7.1
Done installing documentation for bson, mongo (22 sec).
2 gems installed
% /tmp/local/bin/gem install psych
Fetching: psych-1.3.4.gem (100%)
Building native extensions. This could take a while...
Successfully installed psych-1.3.4
Parsing documentation for psych-1.3.4
Installing ri documentation for psych-1.3.4
Done installing documentation for psych (4 sec).
1 gem installed
% /tmp/local/bin/gem cleanup
Cleaning up installed gems...
Attempting to uninstall psych-1.3.4
Successfully uninstalled psych-1.3.4
Attempting to uninstall mongo-1.7.1
Unable to uninstall mongo-1.7.1:
Gem::InstallError: mongo is not installed in GEM_HOME, try:
gem uninstall -i /tmp/local/lib/ruby/gems/2.0.0 mongo
Clean Up Complete