I've added a spec to illustrate the behavior. The spec fails with:

Failures:

1) Mongoid::Relations::Embedded::Many when the embedded relation sorts on a boolean orders by id
Failure/Error: circuit.reload.buses.asc(:id).should eq([ bus_one, bus_two ])
ArgumentError:
comparison of Bus with Bus failed

1. ./lib/mongoid/contextual/memory.rb:379:in `sort!'
2. ./lib/mongoid/contextual/memory.rb:379:in `block in in_place_sort'
3. ./lib/mongoid/contextual/memory.rb:378:in `each_pair'
4. ./lib/mongoid/contextual/memory.rb:378:in `in_place_sort'
5. ./lib/mongoid/contextual/memory.rb:344:in `apply_sorting'
6. ./lib/mongoid/contextual/memory.rb:168:in `initialize'
7. ./lib/mongoid/contextual.rb:47:in `new'
8. ./lib/mongoid/contextual.rb:47:in `create_context'
9. ./lib/mongoid/contextual.rb:30:in `context'
10. ./lib/mongoid/contextual.rb:18:in `each'
11. ./lib/mongoid/criteria.rb:34:in `entries'
12. ./lib/mongoid/criteria.rb:34:in `=='
13. ./spec/mongoid/relations/embedded/many_spec.rb:3311:in `block (3 levels) in <top (required)>'