Hi,

Some callback circular reference is taking my sleep today.

After destroying a child in a referenced relation and updating the parent in a after_destroy callback triggers the destroyed object validation callback.

I know it is hard to understand, so I put a simple app to reproduce the issue.
https://github.com/felipero/mongoid_broken_destroy

To spoil the problem:

Article.create!(name: 'Everything was fine before master from mongoid', comments: [Comment.new])
comment = Comment.first
comment.destroy

results in:

RuntimeError:
       can't modify frozen BSON::Document

Cloning the demo app and running the specs will show this behavior.