http://docs.mongodb.org/manual/release-notes/2.6/#aggregation-pipeline-changes

Syntax in example has flaws:
db.test.aggregate(
{ $match: {} },
{ $redact: { $cond: [

{ $lt: [ '$level', 3' ] }

,
"$$CONTINUE",
"$$PRUNE" ]
}
}
)

Should be:
db.files.aggregate( [
{ $match: {} },
{ $redact: { $cond: [

{ $lt: [ '$level', 3 ] }

,
"$$CONTINUE",
"$$PRUNE" ]
}
}
] )