If the document contains a Date object and the last field is a non-Date object, mongoimport will fail.

Attempting to import

{"DATE": Date("1987-03-06T00:00:00.000Z"), "NUM": 560000}

results in:

mongoimport --db test --collection <<collection>> --file <<file>>

2015-06-12T10:14:20.287-0700 connected to: 127.0.0.1:27017
2015-06-12T10:14:20.288-0700 Failed: error getting extended BSON for document #0: conversion of JSON type '560000' unsupported
2015-06-12T10:14:20.288-0700 imported 0 documents