In 2.6, the invalidation occurs before performing an in-place document update:

https://github.com/mongodb/mongo/blob/r2.6.7/src/mongo/db/catalog/collection.cpp#L384-L391

Due to a refactor to permit a pluggable storage API, mutation invalidations for in-place document updates now occur after the write. The intention of the validation framework is to notify active queries of writes that are about to happen, not ones that have already occurred.

Affects only version 2.8 mongod configured with the MMAP v1 storage engine.