The fsm test timeseries_agg_out.js runs multiple operations concurrently to some $out aggregations which are responsible for creating a collection called "timeseries_agg_out".

The operations such as shardCollection or collMod work on the resulted timeseries_agg_out collection and they expects the collection to be a timeseries and to exists.

However, even though quite unluckly, in case several $out keep failing, the output timeseries might never be created. 

This can happen in the with_balancer suites as shown by BF-33947. 

In that case, the shardCollection would be the first operation to create timeseries_agg_out which is missing the timeseries fields. The collection would therefore be a normal collection, causing the test to eventually fail.