The map_reduce_drop.js test is causing periodic failures back to v5.0 in concurrency test suites. In v5.0, at least, the issue is that a replSetGetStatus command is called against a mongos connection in implicitly_retry_on_database_drop_pending.js.

I've already tried a couple things:

• SERVER-76955 to exclude the test from multiversion in v5.0 because that's the only place it was reported as failing. But I just ran a patch build that surfaced the same failure in non-multi version test suite in v5.0. Removing from multiversion made the BF a lot quieter in v5.0 where it predominantly occurred, but didn't fix the problem.
• SERVER-76925 to get the real error message that our testing infrastructure was previously swallowing. Only just put into master, not older branches.

I recommend re-blacklisting map_reduce_drop.js until further time can be spent on the issue.