In the following example the method update_offices_people_count won't be called when office_id changes, because it gets overwritten by the second line:

after_save :update_offices_people_count, :if => :office_id_changed? # is overwritten …
after_save :update_offices_people_count, :if => :trashed_changed? # … by this line

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Instead write:

after_save :update_offices_people_count, :if => :office_people_count_needs_update?

private

def office_people_count_needs_update?
  office_id_changed? || trashed_changed?
end

Or, move the conditions into the callback. This also allows you test the conditions more easily using a unit test:

after_save :update_offices_people_count

private

def update_offices_people_count
  if office_id_changed? || trashed_changed?  
    ...
  end
end

Also see the card on testing conditional validations.