when rapidly provisioning an ec2 instances through the Foreman UI errors can be produced and the instances fail to build. Taking more time to provision the instance fixes the error.

Output from production.log

Started POST "/hosts" for 195.99.195.90 at 2013-08-29 09:23:03 +0000
    Processing by HostsController#create as */*
      Parameters: {"utf8"=>"✓", "authenticity_token"=>"4nbu87r38iQRKIVtHX+WrVVMKfr11dLepvCTRMD3cVk=", "host"=>{"name"=>"foreman-test11", "compute_resource_id"=>"2", "hostgroup_id"=>"", "environment_id"=>"11", "puppet_ca_proxy_id"=>"1", "puppet_proxy_id"=>"1", "puppetclass_ids"=>[""], "managed"=>"true", "progress_report_id"=>"[FILTERED]", "type"=>"Host::Managed", "mac"=>"", "domain_id"=>"2", "architecture_id"=>"1", "operatingsystem_id"=>"1", "provision_method"=>"image", "build"=>"1", "medium_id"=>"1", "ptable_id"=>"1", "disk"=>"", "root_pass"=>"[FILTERED]", "is_owned_by"=>"2-Users", "enabled"=>"1", "comment"=>"", "overwrite"=>"false"}, "capabilities"=>"image"}
    Failed to save: Mac can't be blank, Mac is invalid, Ip is invalid
      Rendered hosts/_progress.erb (0.7ms)
      Rendered puppetclasses/_selectedClasses.html.erb (0.0ms)
      Rendered puppetclasses/_classes.html.erb (24.4ms)
      Rendered puppetclasses/_class_selection.html.erb (32.7ms)
      Rendered hosts/_compute.html.erb (1.8ms)
      Rendered common/os_selection/_architecture.html.erb (9.1ms)
      Rendered common/os_selection/_operatingsystem.html.erb (158.5ms)
      Rendered hosts/_operating_system.html.erb (199.5ms)
      Rendered hosts/_unattended.html.erb (217.2ms)
      Rendered puppetclasses/_class_parameters.html.erb (0.0ms)
      Rendered puppetclasses/_classes_parameters.html.erb (10.9ms)
      Rendered common_parameters/_inherited_parameters.erb (2.9ms)
      Rendered common_parameters/_puppetclass_parameter.erb (4.0ms)
      Rendered common_parameters/_puppetclasses_parameters.erb (15.7ms)
      Rendered common_parameters/_parameter.erb (2.6ms)
      Rendered common_parameters/_parameters.erb (20.7ms)
      Rendered hosts/_form.html.erb (423.5ms)
      Rendered hosts/new.html.erb within layouts/application (425.5ms)
      Rendered home/_user_dropdown.html.erb (1.4ms)
    Read fragment views/tabs_and_title_records-2 (0.2ms)
      Rendered home/_topbar.html.erb (1.8ms)
    Completed 200 OK in 1116ms (Views: 409.3ms | ActiveRecord: 48.7ms)

Error in UI, New Host creation "Operating System" tab

'foreman-test11.eu-west-1.compute.internal' not found on 'apollo-dev-eu-west (eu-west-1-EC2)'
'foreman-test11.eu-west-1.compute.internal' could be deleted or 'apollo-dev-eu-west (eu-west-1-EC2)' is not responding.