mongo estimates the "max" reasonable chunk size and warns if that chunk size exceeds that, in a effort to detect low cardinality keys. The error message is:

Sat May 17 16:10:09.335 [conn164120] warning: chunk is larger than 1024 bytes because of key { field1: "value1", field2: value2 }

A more informative error message like: "warning possible low cardinality for shard key detected <database>.<collection>" would be helpful.