When Mongoid uses thread-local storage it prefixes the key names with [mongoid], except for :session which for some reason is not prefixed:

lib/mongoid/threaded.rb:      Thread.current[:session] = session
lib/mongoid/threaded.rb:      Thread.current[:session]
lib/mongoid/threaded.rb:      Thread.current[:session] = nil

These should be prefixed like all the other thread-local keys.