Chunk error when upgraded to mongodb 2.6 while uploading doc
Did you follow the guidelines? 


  To upgrade an existing MongoDB deployment to 2.6, you must be running
  2.4. If you’re running a version of MongoDB before 2.4, you must upgrade to 2.4 before upgrading to 2.6.


I have upgraded from 2.4 to 2.6 with no issues on a sharded cluster.