Hashed index causes 'Overflow hashed AND stage buffered data usage' error

I have documents like:

{

  "_id": ObjectId("4cf567c4e4b08fffc3947252"),

  "kodoId": 1167286,

  "p": "/term/Japanese/Tanzania",

  "pl": "/term/japanese/tanzania",

  "r": "/viewWord.jsp?wordId=12083307",

  "l": "eng",

  "cd": new Date("2010-11-30T22:08:20+0100")

}

and a hashed index on 'pl' added yesterday. Some unit tests now fail with:

Overflow hashed AND stage buffered data usage of 33554468 bytes exceeds internal limit of 33554432 bytes

on query

with sort

for some reason if I switch to sort {_id: -1} the error goes away.