There is an error

indexes of target collection read_pref_cmd.mrOut changed during processing. 

which happens time to time during execution of read_pref_cmd.js.

In all cases it happens in mapReduce_noninline_unsharded_ test, which is executed after mapReduce_noninline_sharded_.

It's possible that $out reads an existing index of mrOut created on previous step from lagged secondary.

Solution: use different mrOut collections for different tests.