Every drive comes with a "reserve". If there is a bad sector, the drive tries to re-allocate that sector. Basically it tries to save the drive.
While this seem like a solution, it's just a dirty workaround.
Once this happens, you should always get a new replacement.
From the SMART Wikipedia article:
Count of reallocated sectors. When the hard drive finds a
read/write/verification error, it marks that sector as "reallocated"
and transfers data to a special reserved area (spare area). This
process is also known as remapping, and reallocated sectors are called
"remaps". The raw value normally represents a count of the bad sectors
that have been found and remapped. Thus, the higher the attribute
value, the more sectors the drive has had to reallocate. This allows a
drive with bad sectors to continue operation; however, a drive which
has had any reallocations at all is significantly more likely to fail
in the near future. While primarily used as a metric of the life
expectancy of the drive, this number also affects performance. As the
count of reallocated sectors increases, the read/write speed tends to
become worse because the drive head is forced to seek to the reserved
area whenever a remap is accessed. A workaround which will preserve
drive speed at the expense of capacity is to create a disk partition
over the region which contains remaps and instruct the operating
system to not use that partition.