Steps to reproduce in the shell:

db.bug.save({toString : {} });
db.bug.find();

Resulting error message: "TypeError: x.toString is not a function"

You don't get any results and the only way to fix it in the shell is either db.bug.remove(); or db.bug.drop();

I know it's a stupid object to save, but a bug in my code saved an object with {toString : {} } and I ran into this.

Throw this in there:

(typeof(x.toString) === 'function') ? x.toString() : x

Thanks.