Query is:
db.ActivityEvent.update(

,
{$set: {className:"com.activitystream.common.dto.News"}}, false, true);

Collection has 700k entries and the raw bson is about 685MB

If we turn journaling off the query completes and mongo doesn't die.

Attached mongodb.log. Pertinent lines:
Mon Jul 18 21:37:43 [conn88] notifications_uat Assertion failure a <= 256*1024*1024 util/alignedbuilder.cpp 51

Mon Jul 18 21:37:43 [conn88] dbexception in groupCommit causing immediate shutdown: 0 assertion util/alignedbuilder.cpp:52
Mon Jul 18 21:37:43 Got signal: 6 (Aborted).