I had a dying drive that smart thought until it totally disappeared was a good drive, and actually all parameters did look fine but this system was causing my system to lockup and other bad behavior:

=== START OF INFORMATION SECTION ===
Device Model:     WDC WD20EARS-00MVWB0
Serial Number:    WD-WMAZ20139
Firmware Version: 50.0AB50
User Capacity:    2,000,398,934,016 bytes
Device is:        Not in smartctl database [for details use: -P showall]
ATA Version is:   8
ATA Standard is:  Exact ATA specification draft version not indicated
Local Time is:    Fri Jun  3 09:53:36 2011 PDT
SMART support is: Available - device has SMART capability.
SMART support is: Enabled

=== START OF READ SMART DATA SECTION ===
SMART overall-health self-assessment test result: PASSED

General SMART Values:
Offline data collection status:  (0x84)    Offline data collection activity
                    was suspended by an interrupting command from host.
                    Auto Offline Data Collection: Enabled.
Self-test execution status:      (   0)    The previous self-test routine completed
                    without error or no self-test has ever
                    been run.
Total time to complete Offline
data collection:          (35400) seconds.
Offline data collection
capabilities:              (0x7b) SMART execute Offline immediate.
                    Auto Offline data collection on/off support.
                    Suspend Offline collection upon new
                    command.
                    Offline surface scan supported.
                    Self-test supported.
                    Conveyance Self-test supported.
                    Selective Self-test supported.
SMART capabilities:            (0x0003)    Saves SMART data before entering
                    power-saving mode.
                    Supports SMART auto save timer.
Error logging capability:        (0x01)    Error logging supported.
                    General Purpose Logging supported.
Short self-test routine
recommended polling time:      (   2) minutes.
Extended self-test routine
recommended polling time:      ( 255) minutes.
Conveyance self-test routine
recommended polling time:      (   5) minutes.
SCT capabilities:            (0x3035)    SCT Status supported.
                    SCT Feature Control supported.
                    SCT Data Table supported.

SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x002f   200   200   051    Pre-fail  Always       -       0
  3 Spin_Up_Time            0x0027   191   170   021    Pre-fail  Always       -       5416
  4 Start_Stop_Count        0x0032   100   100   000    Old_age   Always       -       12
  5 Reallocated_Sector_Ct   0x0033   200   200   140    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x002e   200   200   000    Old_age   Always       -       0
  9 Power_On_Hours          0x0032   091   091   000    Old_age   Always       -       6833
 10 Spin_Retry_Count        0x0032   100   253   000    Old_age   Always       -       0
 11 Calibration_Retry_Count 0x0032   100   253   000    Old_age   Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   000    Old_age   Always       -       11
192 Power-Off_Retract_Count 0x0032   200   200   000    Old_age   Always       -       6
193 Load_Cycle_Count        0x0032   042   042   000    Old_age   Always       -       475936
194 Temperature_Celsius     0x0022   117   099   000    Old_age   Always       -       33
196 Reallocated_Event_Count 0x0032   200   200   000    Old_age   Always       -       0
197 Current_Pending_Sector  0x0032   200   200   000    Old_age   Always       -       1
198 Offline_Uncorrectable   0x0030   200   200   000    Old_age   Offline      -       1
199 UDMA_CRC_Error_Count    0x0032   200   200   000    Old_age   Always       -       0
200 Multi_Zone_Error_Rate   0x0008   200   200   000    Old_age   Offline      -       2

SMART Error Log Version: 1
No Errors Logged

SMART Self-test log structure revision number 1
No self-tests have been logged.  [To run self-tests, use: smartctl -t]


SMART Selective self-test log data structure revision number 1
 SPAN  MIN_LBA  MAX_LBA  CURRENT_TEST_STATUS
    1        0        0  Not_testing
    2        0        0  Not_testing
    3        0        0  Not_testing
    4        0        0  Not_testing
    5        0        0  Not_testing
Selective self-test flags (0x0):
  After scanning selected spans, do NOT read-scan remainder of disk.
If Selective self-test is pending on power-up, resume after 0 minute delay.


At the end smart did catch it, but just before it died:

un  1 17:47:52 box10 smartd[8529]: Device: /dev/sda, opened
Jun  1 17:47:52 box10 smartd[8529]: Device /dev/sda: using '-d sat' for ATA disk behind SAT layer.
Jun  1 17:47:52 box10 smartd[8529]: Device: /dev/sda, opened
Jun  1 17:47:52 box10 smartd[8529]: Device: /dev/sda, not found in smartd database.
Jun  1 17:47:52 box10 smartd[8529]: Device: /dev/sda, is SMART capable. Adding to "monitor" list.
Jun  1 17:47:53 box10 smartd[8529]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 17:47:53 box10 smartd[8529]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 18:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 18:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 18:47:53 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 18:47:53 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 19:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 19:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 19:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 19:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 20:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 20:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 20:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 20:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 21:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 21:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 21:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 21:47:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 22:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 22:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 22:47:53 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 22:47:53 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 23:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors
Jun  1 23:17:54 box10 smartd[8546]: Device: /dev/sda, 1 Offline uncorrectable sectors
Jun  1 23:47:53 box10 smartd[8546]: Device: /dev/sda, 1 Currently unreadable (pending) sectors