1.9.3p0 :001 > require 'quantity'
1.9.3p0 :002 > Quantity::VERSION
1.9.3p0 :003 > [1.gallon, 1.gallon].flatten
ArgumentError: Unknown target unit type: ary
from /home/brundage/.rvm/gems/ruby-1.9.3-p0/gems/quantity-0.1.2/lib/quantity.rb:356:in `method_missing'
from (irb):3:in `flatten'
from /home/brundage/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>'
This would be a result of Ruby 1.9's new flattening behavior. Quantity is incorrectly assuming here that to_* is a unit conversion attempt and failing; it should probably return NoMethodError instead, which would fix the flattening and probably some other unknown bugs. See http://yehudakatz.com/2010/01/02/the-craziest-fing-bug-ive-ever-seen/
In other news, I don't really work on this anymore, but am happy to give you gem push rights if you're running with it.
I patched it by calling super. I'll change to NoMethodError & submit a pull.
Thanks, I'll take gem push rights. I'm brundage on rubygems. Would you add me as a collaborator on this repo too?