My /etc/mdadm/mdadm.conf looks like this:

DEVICE          /dev/sda5 /dev/sdb5 /dev/sdc5
ARRAY           /dev/md0 devices=/dev/sda5,/dev/sdb5,/dev/sdc5

MAILADDR root

And my /etc/raidtab looks like this:

raiddev /dev/md0
        raid-level      5
        nr-raid-disks   3
        nr-spare-disks  2
        persistent-superblock 1
        parity-algorithm        left-symmetric
        chunk-size      32
        device          /dev/sda5
        raid-disk       0
        device          /dev/sdb5
        raid-disk       1
        device          /dev/sdc5
        raid-disk       2

If I try to assemble the array I get:

root@magnia:~# mdadm -A /dev/md0 /dev/sd[abc]5 /dev/sd[de]
mdadm: superblock on /dev/sdd doesn't match others - assembly aborted