Comment 8 for bug 823366

I think you are trying to use a deprecated and unsupported means of bringing up the bonded interface in /etc/network/interfaces. As explained in /usr/share/doc/ifenslave-2.6/README.Debian.gz, the bonding interface needs to be brought up via stanzas for each of the physical interfaces. For example:

iface bond0 inet static
  bond_slaves none
  bond_primary eth2
  bond_mode 1
  bond_arp_interval 2000
  bond_arp_ip_target 192.168.100.1
  address 192.168.100.50
  netmask 255.255.255.192
  gateway 192.168.100.1

auto eth0
iface eth0 inet manual
  bond_primary eth2
  bond_master bond0

auto eth2
iface eth2 inet manual
  bond_primary eth2
  bond_master bond0

If the example you provided was working before as a complete /etc/network/interfaces, I think it did so by chance alone.