The 'build' button for a host should not give a successful reply if the TFTP proxy does not:
1) The TFTP proxy for a given subnet is not defined.
2) As a result of #1 the required images for /var/lib/tftpboot/boot are not downloaded

#1 This actually constitutes an unlogged error in for Foreman. I have 2 copies of Foreman. In the working copy I get this in the production.log:
Started GET "/hosts/kvm3.edsonmanners.com/setBuild" for 192.168.5.3 at 2013-12-20 11:41:23 -0500
Processing by HostsController#setBuild as HTML
Parameters: {"id"=>"kvm3.edsonmanners.com"}
Add the TFTP configuration for kvm3.edsonmanners.com
Fetching required TFTP boot files for kvm3.edsonmanners.com
Redirected to https://localhost:8000/hosts/kvm3.edsonmanners.com
Completed 302 Found in 590ms (ActiveRecord: 47.3ms)

In the copy of foreman without the TFTP proxy defined in my subnet I get nothing. No error saying:
"TFTP Proxy for subnet xxxx for host yyyyy is not defined." This should also be added as one of these red windows in the Foreman GUI.