To reproduce, start a long-running script that inserts large documents into a collection.

Then run:

ab -n10000 -c2 http://127.0.0.1:28017/

You'll get assertions like these. Not sure how to explain this, but it's definitely triggered by hitting the web server.

Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:64941
Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:65073
Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:65201
Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:65328
Tue Mar 9 11:30:34 Assertion: 10330:Element extends past end of object
0x10005b7d7 0x10000d4e8 0x10000d70f 0x100127cf3 0x10012cbd4 0x10016925e 0x100169ae6 0x100125357 0x1001eca64 0x7fff86d52f8e 0x7fff86d52e41
0 mongod 0x000000010005b7d7 _ZN5mongo11msgassertedEiPKc + 455
1 mongod 0x000000010000d4e8 _ZNK5mongo7BSONObj8toStringEv + 280
2 mongod 0x000000010000d70f _ZN5mongolsERSoRKNS_7BSONObjE + 31
3 mongod 0x0000000100127cf3 _ZN5mongo11DbWebServer15doUnlockedStuffERSt18basic_stringstreamIcSt11char_traitsIcESaIcEE + 1683
4 mongod 0x000000010012cbd4 _ZN5mongo11DbWebServer9doRequestEPKcSsRSsRiRSt6vectorISsSaISsEERKNS_8SockAddrE + 788
5 mongod 0x000000010016925e _ZN5mongo13MiniWebServer8acceptedEiRKNS_8SockAddrE + 238
6 mongod 0x0000000100169ae6 _ZN5mongo13MiniWebServer3runEv + 854
7 mongod 0x0000000100125357 _ZN5mongo15webServerThreadEv + 711
8 mongod 0x00000001001eca64 thread_proxy + 132
9 libSystem.B.dylib 0x00007fff86d52f8e _pthread_start + 331
10 libSystem.B.dylib 0x00007fff86d52e41 thread_start + 13
Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:65457
Tue Mar 9 11:30:34 MiniWebServer: connection accepted from 127.0.0.1:49200