After reconfiguring one of the shard replica sets, mongos started failing with the following (once every 1-3 seconds) and consumed 100% CPU:
Tue Dec 25 17:43:56 [conn67701] Assertion failure n == a src/mongo/s/shard.h 105 0x80e931 0x7d810d 0x6aa7d9 0x6c1d58 0x73071f 0x732724 0x769f88 0x76a7c5 0x76c415 0x771d59 0x76f560 0x770a52 0x56ce2e 0x58371c 0x589c 79 0x7815a2 0x75a99b 0x500751 0x7fcb51 0x7fb094207e9a /usr/bin/mongos(_ZN5mongo15printStackTraceERSo+0x21) [0x80e931] /usr/bin/mongos(_ZN5mongo12verifyFailedEPKcS1_j+0xfd) [0x7d810d] /usr/bin/mongos(_ZN5mongo17ChunkRangeManager12_insertRangeESt23_Rb_tree_const_iteratorISt4pairIKNS_7BSONObjEN5boost10shared_ptrIKNS _5ChunkEEEEESB_+0x539) [0x6aa7d9] /usr/bin/mongos(_ZN5mongo12ChunkManager18loadExistingRangesERKSs+0x788) [0x6c1d58] /usr/bin/mongos(_ZN5mongo8DBConfig15getChunkManagerERKSsbb+0x51f) [0x73071f] /usr/bin/mongos(_ZN5mongo8DBConfig23getChunkManagerIfExistsERKSsbb+0x34) [0x732724] /usr/bin/mongos(_ZN5mongo17checkShardVersionEPNS_12DBClientBaseERKSsN5boost10shared_ptrIKNS_12ChunkManagerEEEbi+0x778) [0x769f88] /usr/bin/mongos(_ZN5mongo17checkShardVersionEPNS_12DBClientBaseERKSsN5boost10shared_ptrIKNS_12ChunkManagerEEEbi+0xfb5) [0x76a7c5] /usr/bin/mongos(_ZN5mongo14VersionManager19checkShardVersionCBEPNS_12DBClientBaseERKSsbi+0x35) [0x76c415] /usr/bin/mongos(_ZN5mongo17ClientConnections13checkVersionsERKSs+0x149) [0x771d59] /usr/bin/mongos(_ZN5mongo15ShardConnection5_initEv+0x2c0) [0x76f560] /usr/bin/mongos(_ZN5mongo15ShardConnectionC1ERKNS_5ShardERKSsN5boost10shared_ptrIKNS_12ChunkManagerEEE+0xa2) [0x770a52] /usr/bin/mongos(_ZN5mongo27ParallelSortClusteredCursor28setupVersionAndHandleSlaveOkEN5boost10shared_ptrINS_23ParallelConnectionSta teEEERKNS_5ShardENS2_IS5_EERKNS_15NamespaceStringERKSsNS2_IKNS_12ChunkManagerEEE+0x2ee) [0x56ce2e] /usr/bin/mongos(_ZN5mongo27ParallelSortClusteredCursor9startInitEv+0xd3c) [0x58371c] /usr/bin/mongos(_ZN5mongo27ParallelSortClusteredCursor8fullInitEv+0x9) [0x589c79] /usr/bin/mongos(_ZN5mongo13ShardStrategy7queryOpERNS_7RequestE+0x472) [0x7815a2] /usr/bin/mongos(_ZN5mongo7Request7processEi+0x1fb) [0x75a99b] /usr/bin/mongos(_ZN5mongo21ShardedMessageHandler7processERNS_7MessageEPNS_21AbstractMessagingPortEPNS_9LastErrorE+0x71) [0x500751] /usr/bin/mongos(_ZN5mongo3pms9threadRunEPNS_13MessagingPortE+0x411) [0x7fcb51] /lib/x86_64-linux-gnu/libpthread.so.0(+0x7e9a) [0x7fb094207e9a]
Restarting mongos fixed the problem.