see these errors when running mongo and mongorestore:
mongo(821,0x7fffbc3713c0) malloc: *** malloc_zone_unregister() failed for 0x7fffbc367000
mongorestore(1055,0x7fffbc3713c0) malloc: *** malloc_zone_unregister() failed for 0x7fffbc367000

It may be related to the recent upgrade to Sierra.

System information:

Model Name: MacBook Air
Model Identifier: MacBookAir5,2
Processor Name: Intel Core i7
Processor Speed: 2 GHz
Number of Processors: 1
Total Number of Cores: 2
L2 Cache (per Core): 256 KB
L3 Cache: 4 MB
Memory: 8 GB

System Version: macOS 10.12.1 (16B2555)
Kernel Version: Darwin 16.1.0

*brew info mongodb26:

homebrew/versions/mongodb26: stable 2.6.12 (bottled)
High-performance document-oriented database
https://www.mongodb.org/
/usr/local/Cellar/mongodb26/2.6.11 (17 files, 329.2M)
Poured from bottle on 2016-09-20 at 11:28:43
/usr/local/Cellar/mongodb26/2.6.12 (18 files, 314.8M) *
Poured from bottle on 2016-11-16 at 10:59:05
From: https://github.com/Homebrew/homebrew-versions/blob/master/mongodb26.rb
==> Dependencies
Build: scons ✘
Optional: boost ✘, openssl ✔
==> Requirements
Required: macOS >= 10.6 ✔
==> Options
--with-boost
Compile using installed boost, not the version shipped with mongodb
--with-openssl
Build with openssl support

mongod --config /usr/local/etc/mongod.conf

1. Store data in /usr/local/var/mongodb instead of the default /data/db
   dbpath = /usr/local/var/mongodb

1. Append logs to /usr/local/var/log/mongodb/mongo.log
   logpath = /usr/local/var/log/mongodb/mongo.log
   logappend = true

1. Only accept local connections
   bind_ip = 127.0.0.1