It is impossible to cancel host creation if you edit the templates.

In this case, I changed the partition template used for the OS which should be installed on that machine.
Then I wanted to cancel the build and restart it (to be sure that foreman uses the new partitioning layout).

The log of foreman says:

Started GET "/hosts/sv6011.muc.baag/cancelBuild" for 10.110.10.95 at Wed Jan 18 14:46:02 +0000 2012
  Processing by HostsController#cancelBuild as HTML
  Parameters: {"id"=>"sv6011.muc.baag"}
Failed to save: Ptable Solaris medium disk mirrored does not belong to Solaris 5.10 (1009) operating system
Operation FAILED: undefined method `join' for "Failed to cancel pending build for sv6011.muc.baag":String
Rendered bookmarks/_list.html.erb (1.4ms)
Rendered bookmarks/_list.html.erb (0.4ms)
Rendered home/_settings.html.erb (5.0ms)
Rendered home/_topbar.rhtml (11.4ms)
Rendered common/500.rhtml within layouts/application (18.8ms)
Completed 500 Internal Server Error in 74ms (Views: 29.3ms | ActiveRecord: 4.2ms)