Release notes for 2.6:

In 2.4, we allowed a shard to refresh metadata even when the shard name was unknown to the shard - this is dangerous, and is disallowed in 2.6. However, changing this means that migrations TO 2.6 shards FROM 2.4 shards may not have enough information (i.e. the shard name) to begin - and so may error out with:

warning: cannot start recv'ing chunk [

,

) :: caused by :: cannot refresh metadata for test.collection before shard name has been set

This should not cause orphaned data or any other problems, but it may interfere with migrations in a multi-version cluster (which we do not support anyway, and is very unlikely if the 2.6 shard has any data on it at all).