If your are seeing this one when starting meteor:
Unexpected mongo exit code 100. Restarting.
Can't start mongod
MongoDB had an unspecified uncaught exception.
Check if mongoDB is running already, otherwise, if you can find a file called
.meteor/local/db/mongod.lock means that mongo did not stop properly last time. Just deleting this lock file should fix it. It’s a bit unfortunate that the error message doesn’t give any hint in this direction.