In SERVER-101416 assertions were added to the shard role to ensure that there weren't acquisitions happening that should use lastApplied read source but wouldn't due to a prior acquisition using lastest.

However, we have seen test failures triggering this due to step down - a primary begins an aggregation and so the first acquisition uses read source latest and acquires a snapshot. Then the primary steps down and the second acquisition would say that the read source needs to be changed to lastApplied since we are now operating on a secondary.

This case isn't an actual problem, though, because the snapshot was acquired when the node was a primary and thus it is valid to read it from latest. We would only need to read from lastApplied if we were to acquire a new snapshot.