Users are being tripped up by the caveats related to spherical queries, especially the long/lat order requirement. If you use lat/long order queries will raise an assertion similar to this:

error:

{ "$err" : "assertion db/geo/2d.cpp:1385" }

This is so common that the assert should really give a hint as to what the problem is.

Here's some example code for repro steps:

// Wrong!
switched to db mygeo
> var earthRadius = 6378;
> db.points.insert(

{location: [37.762, -122.441]}

);
> db.points.insert(

{location: [37.804, -122.270]}

);
> db.points.ensureIndex(

{location:"2d"}

);
> db.points.find({location:{$nearSphere: [37.762,-122.441], $maxDistance: 1/earthRadius}});
error:

{ "$err" : "assertion db/geo/2d.cpp:1385" }

// Correct

> db.points.drop()
true
> db.points.insert(

{location: [-122.441, 37.762]}

);
> db.points.insert(

{location: [-122.270, 37.802]}

);
> db.points.ensureIndex(

{location:"2d"}

);
> db.points.find({location:{$nearSphere: [-122.441, 37.762], $maxDistance: 1/earthRadius}});

{ "_id" : ObjectId("4dc2e010c545b10a39a72bb3"), "location" : [ -122.441, 37.762 ] }