On attempting to build a background index on {_id:-1,random:-1}, I immediately got:

Tue Mar 8 20:55:09 [conn19] insert test.c 1362ms
200/22688 0%
Tue Mar 8 20:55:13 [conn24] getmore local.oplog.rs cid:8917473787021710562 getMore: { ts:

{ $gte: new Date(5581772255597166593) }

} bytes:6786876 nreturned:2 7134ms
Tue Mar 8 20:55:13 [conn44] test.system.indexes Assertion failure _a != -1 db/pdfile.h 427
0x55603e 0x567a41 0x784eb8 0x796883 0x788603 0x78a30b 0x78b78f 0x76890d 0x76a568 0x8bfb5e 0x8d0e10 0x7fbb0e90ca04 0x7fbb0decad4d
./mongod(_ZN5mongo12sayDbContextEPKc+0xae) [0x55603e]
./mongod(_ZN5mongo8assertedEPKcS1_j+0x111) [0x567a41]
./mongod [0x784eb8]
./mongod(_ZN5mongo23BackgroundIndexBuildJob18addExistingToIndexEPKcPNS_16NamespaceDetailsERNS_12IndexDetailsEi+0x313) [0x796883]
./mongod [0x788603]
./mongod(_ZN5mongo11DataFileMgr6insertEPKcPKvibRKNS_11BSONElementEb+0xf4b) [0x78a30b]
./mongod(_ZN5mongo11DataFileMgr16insertWithObjModEPKcRNS_7BSONObjEb+0x5f) [0x78b78f]
./mongod(_ZN5mongo14receivedInsertERNS_7MessageERNS_5CurOpE+0x37d) [0x76890d]
./mongod(_ZN5mongo16assembleResponseERNS_7MessageERNS_10DbResponseERKNS_8SockAddrE+0x1908) [0x76a568]
./mongod(_ZN5mongo10connThreadEPNS_13MessagingPortE+0x21e) [0x8bfb5e]
./mongod(thread_proxy+0x80) [0x8d0e10]
/lib/libpthread.so.0 [0x7fbb0e90ca04]
/lib/libc.so.6(clone+0x6d) [0x7fbb0decad4d]
Tue Mar 8 20:55:14 [conn44] test.system.indexes caught assertion indexRecord test.c.$id-1_random_-1 _id: ObjectId('4d757070a8ef85c41e00013d')

Over and over. It's only present on one the primary of one shard out of 3, all running as durable replica sets.