When $all is combined with other operators (ie: $in, $gt, $lt), the results are not the expected.

Example:
Having these 9 documents:

{ n: 1, x: ["a","a"] } { n: 2, x: ["a","b"] } { n: 3, x: ["a","c"] } { n: 4, x: ["b","a"] } { n: 5, x: ["b","b"] } { n: 6, x: ["b","c"] } { n: 7, x: ["c","a"] } { n: 8, x: ["c","b"] } { n: 9, x: ["c","c"] }

> db.x.find({x:{ $in: ["a","b"] }}); return all the documents
having "a" or "b" as values of the x array - 8 documents (just
the 9th document is excluded).

> db.x.find({x:{ $all: ["c"] }}); return all the documents
having "c" as value of the x array, - 5 documents (the 3, 6, 7,
8 and 9).

> db.x.find({x:{ $in: ["a","b"], $all: ["c"] }}); should return all the documents having "a" or "b" as values and having "c" as value (the 3, 6, 7 and 8) - the intersection of both queries. Instead, return nothing.

mongodb-user theread: http://groups.google.com/group/mongodb-user/browse_thread/thread/4033cd8335f7261e