Test:

t.save( { a:{ '':1 } } );
t.update( {}, { $set:{ 'a.b':2 } } );
printjson( db.getLastError() );
printjson( t.findOne() );

Documents or subdocuments with empty string keys can cause $set updates to that document to fail.

Specifically, $set ing a new field at the same level as an empty-string key, will silently fail. Same for updates to children of siblings to empty-string key ("nephew/niece" fields).

This behaviour sometimes changes depending on the number of updates inside $set.

The test script output should make it clear enough:

• • • Single - $set updates *** //php test.php 1 multi_tests
      [...]

      	*** TEST 3 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.m.new":1}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"new":1,"rock":1},"o":{"funny":2}}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      	Test FAILED
      

      	*** TEST 4 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.p.p":10}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"p":{"p":10}}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      	Test FAILED
      

      	*** TEST 5 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.p":1}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"p":1}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      	Test FAILED
      

      	*** TEST 6 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.other":10}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"other":10}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      	Test FAILED
      

      *** Multi - $set updates *** //php test.php 1 multi_tests
      [...]

      	*** TEST 3 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.m.new":1,"tags.m.newer":1}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"new":1,"newer":1,"rock":1},"o":{"funny":2}}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"new":1,"newer":1,"rock":1},"o":{"funny":2}}}
      	Test passed //no problem creating two new tags (one had failed)
      

      	*** TEST 4 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.p.p":10,"tags.p.q":11}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"p":{"p":10,"q":11}}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"p":{"p":10,"q":11}}}
      	Test passed
      

      	*** TEST 5 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.p":1,"tags.q":1}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"p":1,"q":1}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"q":1}}
      	Test FAILED //note one update made it through
      

      	*** TEST 6 ***
      original:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2}}}
      update: 	{"$set":{"tags.other":10,"tags.music":11}}
      expected:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"music":11,"o":{"funny":2},"other":10}}
      received:	{"_id":"1","tags":{"":{"oops":1},"m":{"rock":1},"o":{"funny":2},"other":10}}
      	Test FAILED //note one update made it through
      

If the empty-string fields are removed from the original object, all tests succeed (run with 0 instead of 1)

Attached tests are PHP (PHPMongo 1.2.9) but behaviour is the same on the shell.