From this thread: https://groups.google.com/forum/?hl=en&fromgroups=#!topic/mongodb-user/i8Te7ip2T-k

I have had a sharded DB with a single shard for a while. When I added a second shard on Monday, 3 of the chunks failed to migrate - and they are in a continuous loop of starting migrating, fail (invalid modified specified) every few seconds.

(In addition the chunk mentioned in the above thread, there are 2 other chunks that behave the same way except that they don't generate any errors, they just abort in step 2 without explanation)

gist with some more relevant logging (starting from when I added the second shard): https://gist.github.com/Alex-Ikanow/5008868