For example:

c = t.find( {$or:[

{a:2}

,

{b:3}

,

{c:4}

]} ).batchSize( 2 );
c.next();
t.remove(

{b:3}

);
assert.eq.automsg( "3", c.itcount() );

The first getmore, caused by itcount(), may happen either before or after the remove if mediated by mongos. The following workaround is currently needed:

c = t.find( {$or:[

{a:2}

,

{b:3}

,

{c:4}

]} ).batchSize( 2 );
c.next();
t.remove(

{b:3}

);
db.getLastError();
assert.eq.automsg( "3", c.itcount() );

Here is an example logging expected behavior from the above script:

m30001| Mon Apr 25 23:31:42 [conn4] query test.jstests_or5 ntoreturn:2 reslen:102 nreturned:2 0ms
m30001| Mon Apr 25 23:31:42 [conn4] remove test.jstests_or5 query:

{ b: 3.0 }

0ms
m30001| Mon Apr 25 23:31:42 [conn8] getmore test.jstests_or5 cid:4546250274389464629 ntoreturn:2 getMore: { $or: [

{ a: 2.0 }

,

{ b: 3.0 }

,

{ c: 4.0 }

] } bytes:86 nreturned:2 0ms
m30001| Mon Apr 25 23:31:42 [conn8] getmore test.jstests_or5 cid:4546250274389464629 ntoreturn:2 getMore: { $or: [

{ a: 2.0 }

,

{ b: 3.0 }

,

{ c: 4.0 }

] } bytes:20 nreturned:0 0ms

Here is an example of unexpected behavior that sometimes occurs:

m30001| Mon Apr 25 23:11:51 [conn4] query test.jstests_or5 ntoreturn:2 reslen:102 nreturned:2 0ms
m30001| Mon Apr 25 23:11:51 [conn8] getmore test.jstests_or5 cid:5426795093500586304 ntoreturn:2 getMore: { $or: [

{ a: 2.0 }

,

{ b: 3.0 }

,

{ c: 4.0 }

] } bytes:86 nreturned:2 0ms
m30001| Mon Apr 25 23:11:51 [conn8] getmore test.jstests_or5 cid:5426795093500586304 ntoreturn:2 getMore: { $or: [

{ a: 2.0 }

,

{ b: 3.0 }

,

{ c: 4.0 }

] } bytes:86 nreturned:2 0ms
m30001| Mon Apr 25 23:11:51 [conn8] getmore test.jstests_or5 cid:5426795093500586304 ntoreturn:2 getMore: { $or: [

{ a: 2.0 }

,

{ b: 3.0 }

,

{ c: 4.0 }

] } bytes:20 nreturned:0 0ms
m30001| Mon Apr 25 23:11:51 [conn4] remove test.jstests_or5 query:

{ b: 3.0 }

0ms

As far as I know there is a single connection from mongo to mongos in these examples, but there seem to be two connections from mongos to mongod and this causes the unexpected operation ordering.