a key value inserted into this  index  may  exceed  the  maximum
  length

  Explanation:  It is  possible  that  when  the  key  compression
  algorithm  is  run  for some values inserted into this index the
  resulting key length may exceed 255 bytes.   This  condition  is
  dependent on the data values and will probably occur rarely.

  User Action:  This is just a warning.  If you drop the index and
  recreate it as an uncompressed index you will guarantee that all
  rows can be stored.