If you load aws with warnings turned on, you'll see this:
/home/phiggins/projects/aws/lib/ec2/ec2.rb:1045: warning: method redefined; discarding old describe_availability_zones
/home/phiggins/projects/aws/lib/ec2/ec2.rb:407: warning: previous definition of describe_availability_zones was here
If you load aws with warnings turned on, you'll see this: