$text queries against a compound text index pass an incorrect predicate to the index, unless the index isn't multikey (i.e. all documents have at most one indexable term).

Reproduce with the following:

function runTest(textData) {
    db.foo.drop();
    db.foo.insert({a: 17, b: "irrelevant"});
    db.foo.insert({a: 17, b: textData});
    db.foo.ensureIndex({a: 1, b: "text"});
    return db.foo.count({a: 17, $text: {$search: "foo"}});
}

print(runTest("foo")); // correct: outputs 1 (text index is not multikey)
print(runTest("foo bar")); // incorrect: outputs 2 (text index is multikey)

Output for the "correct" case:

2014-01-13T19:56:12.455-0500 [conn3] enumerator received root:
$and
    a $gt 0.0 First: 0 notFirst: full path: a
    TEXT : query=hello, language = , tag=First: notFirst: 0 full path: _fts

2014-01-13T19:56:12.455-0500 [conn3] Tagging memoID 0
2014-01-13T19:56:12.455-0500 [conn3] Enumerator: memo right before moving:
2014-01-13T19:56:12.455-0500 [conn3] [Node #0]: AND enumstate counter 0
choice 0:
	subnodes:
	idx[0]
		pos 0 pred a $gt 0.0

		pos 1 pred TEXT : query=hello, language = , tag=NULL

Output for the "incorrect" case:

2014-01-13T19:55:46.798-0500 [conn2] enumerator received root:
$and
    a $gt 0.0 First: 0 notFirst: full path: a
    TEXT : query=hello, language = , tag=First: notFirst: 0 full path: _fts

2014-01-13T19:55:46.798-0500 [conn2] Tagging memoID 0
2014-01-13T19:55:46.798-0500 [conn2] Enumerator: memo right before moving:
2014-01-13T19:55:46.798-0500 [conn2] [Node #0]: AND enumstate counter 0
choice 0:
	subnodes:
	idx[0]
		pos 0 pred a $gt 0.0

Original description below.

So, given a collection inventory :

   { _id: 1, dept: "tech", description: "a fun green computer" }
   { _id: 2, dept: "tech", description: "a wireless red mouse" }
   { _id: 3, dept: "kitchen", description: "a green placemat" }
   { _id: 4, dept: "kitchen", description: "a red peeler" }
   { _id: 5, dept: "food", description: "a green apple" }
   { _id: 6, dept: "food", description: "a red potato" }

and the following compound index:

db.inventory.ensureIndex( { dept: 1, description: "text" } )

The following query seems to be 'or'ing the conditions or something

>    db.inventory.find( { dept: {$in:[ "kitchen", "food" ] }, $text: { $search: "green" } } )
{ "_id" : 5, "dept" : "food", "description" : "a green apple" }  
{ "_id" : 6, "dept" : "food", "description" : "a red potato" }   // This shouldn't return
{ "_id" : 3, "dept" : "kitchen", "description" : "a green placemat" }
{ "_id" : 4, "dept" : "kitchen", "description" : "a red peeler" }  // this shouldn't return either