New (to me) production errors

Seeing a lot of this today:

NameError (uninitialized constant Sidebars::Plugin):
/vendor/rails/activerecord/lib/…/…/activesupport/lib/
active_support/dependencies.rb:100:in const_missing' /app/controllers/sidebar_controller.rb:38:inavailable_sidebars’
/app/controllers/sidebar_controller.rb:30:in enabled_sidebars' /app/controllers/sidebar_controller.rb:13:indisplay_plugins’
/vendor/rails/actionpack/lib/action_controller/base.rb:910:in
send' /vendor/rails/actionpack/lib/action_controller/base.rb:910:inperform_action_without_filters’
/vendor/rails/actionpack/lib/action_controller/filters.rb:368:in
`perform_action_without_benchmark’
…blah, blah.

Always seems to be followed by

RuntimeError (Controller stack got out of kilter!):
/app/models/blog.rb:116:in `after’
…blah, blah.

Gary