First thing I'd do is unmount the filesystem and run fsck on it. I'd also run something like fdisk, cfdisk, or sfdisk to list the partition/slice layout for the physical disk. Ensure that the starting sector and lengths of each don't overlap. Be sure to read and understand the manual page for the fdisk like command you decide to use.
Depending on how you have your disk arranged, it is quite possible that when partitions/slices were defined, starting points were not correctly determined and data from a partition ahead of the one housing your odd filesystem has overwritten the filesystem in question, or you simply have a disk that's going bad.
Another clue would be any disk related messages in the system log.
I'd say this is inconclusive. Could be a bad spot in a very unfortunate location (the partition/slice table), or could be poorly defined partition start/end points causing the corrupted partition to have been overwritten with data from the adjacent partition. You still need to look at output from an fdisk like tool to ensure that there isn't any overlap (if there is you've likely got much more corruption than what you've found). If there isn't overlap, then my guess would be a bad spot on the disk.
I'm not an expert when it comes to hard disk issues, maybe someone else with a bit more savvy will chime in and lend some help.