One of our Macs got a filesystem error of "invalid node structure." You can still mount the drive (if you want to get at the user data), but the drive is unbootable.
I did a lot of Googling and ran into Please help! Invalid node structure and invalid record count, which contained the solution:
Note: You may get that it didn't work:
volume type is pure HFS+
primary MDB is at block 0 0x00
alternate MDB is at block 0 0x00
primary VHB is at block 2 0x02
alternate VHB is at block 1873113742 0x6fa5728e
sector size = 512 0x200
VolumeObject flags = 0x07
total sectors for volume = 1873113744 0x6fa57290
total sectors for embedded volume = 0 0x00
CheckHFS returned 8, fsmodified = 1
doing update / reload mount for /Volumes/Macintosh HD now
Just to double-check it did work, boot into single-user mode and run the fsck command when prompted.