The elemMatchProjection.js integration test has various assertions which sort by _id, and depend on this sort enforcing a particular ordering. The test, however, does not supply explicit _id values. It instead relies on the shell generating ObjectIds, and it assumes that these ObjectIds are monotically increasing.
This is not a sound assumption. As documented here, the least significant three bytes of an ObjectId consist of an incrementing counter which is seeded with a random value. If the uint32_t seed is close to 0xffffffff, then during the execution of the test, the counter will roll over to zero. If this occurs, the sort order of the _id values will not match the shell's insertion order, causing the test to fail.
The test fails consistently after building the server with the following patch:
This replaces random seeding of the ObjectId counter with a hardcoded value of 0xffffff35, designed to demonstrate the problem.