We found a log of strange warning messages in mongodb.log files on both of our secondary servers:

Sat Aug 11 00:18:30 [rsHealthPoll] warning assertion failure d.size() < 1024 src/mongo/util/concurrency/task.cpp 122

Tue Aug 14 22:35:52 [rsSync] local.oplog.rs warning assertion failure _intents.size() < 2000000 src/mongo/db/dur_commitjob.h 101

Fri Aug 17 17:38:46 [rsSync] local.oplog.rs warning assertion failure _intents.size() < 2000000 src/mongo/db/dur_commitjob.h 101

Fri Aug 17 17:38:53 [journal] warning assertion failure a <= 256*1024*1024 src/mongo/util/alignedbuilder.cpp 90

Here is a full stack trace from one of such messages:

Sat Aug 18 23:21:26 [conn17351] end connection 10.13.34.150:50228 (17 connections now open)
Sat Aug 18 23:21:26 [initandlisten] connection accepted from 10.13.34.150:44379 #17353 (18 connections now open)
Sat Aug 18 23:21:29 [journal] warning assertion failure a <= 256*1024*1024 src/mongo/util/alignedbuilder.cpp 90
0xb400d1 0x68127a 0x792f83 0x6d2635 0x6d2944 0x637bd0 0x6389f9 0x6390a4 0x5f18a9 0x7f6ecec3ed8c 0x7f6ecdfe0c2d
/usr/bin/mongod(_ZN5mongo15printStackTraceERSo+0x21) [0xb400d1]
/usr/bin/mongod(_ZN5mongo9wassertedEPKcS1_j+0x11a) [0x68127a]
/usr/bin/mongod(_ZN5mongo14AlignedBuilder14growReallocateEj+0x63) [0x792f83]
/usr/bin/mongod() [0x6d2635]
/usr/bin/mongod(_ZN5mongo3dur13PREPLOGBUFFERERNS0_11JSectHeaderERNS_14AlignedBuilderE+0x214) [0x6d2944]
/usr/bin/mongod(_ZN5mongo3dur27groupCommitWithLimitedLocksEv+0xa0) [0x637bd0]
/usr/bin/mongod() [0x6389f9]
/usr/bin/mongod(_ZN5mongo3dur9durThreadEv+0x364) [0x6390a4]
/usr/bin/mongod() [0x5f18a9]
/lib/x86_64-linux-gnu/libpthread.so.0(+0x6d8c) [0x7f6ecec3ed8c]
/lib/x86_64-linux-gnu/libc.so.6(clone+0x6d) [0x7f6ecdfe0c2d]
Sat Aug 18 23:21:31 [conn17352] end connection 10.8.233.42:53905 (17 connections now open)
Sat Aug 18 23:21:31 [initandlisten] connection accepted from 10.8.233.42:53907 #17354 (18 connections now open)

Full mongodb.log file attached to this issue.