When you have a pool with failed instances and go to pool family show page and select 'pools' tab, 'failed instances' column displays string representation of ruby array instead of number.

in app/views/pool_families/_pools.html.haml should be:
    %td{:class => 'center'}= pool.statistics[:instances_failed_count]

instead of:
    %td{:class => 'center'}= pool.statistics[:instances_failed]