MongoDB fails when starting Meteor

If you’re 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. If not, check if you can find a file called .meteor/local/db/mongod.lock. It means that MongoDB 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.