Example of failure - https://evergreen.mongodb.com/task/mongodb_mongo_v4.2_enterprise_rhel_62_64_bit_display_sharding_last_stable_mongos_and_mixed_shards_c797110b90fd5d950fa5d78aa26ae198c76883c6_21_12_03_21_30_06

How It Happens:

• The tests are running on a mixed cluster (my guess is 4.2 something and 4.0 something)
• The 4.2 test uses the failpoints introduced in 4.2
• The 4.0 server code does not have the wait for failpoint command (just searched), so that when the 4.2 test uses the failpoint helpers to execute the failpoint commands, it will fail Because the 4.0 shards don't have those command