Not sure if this is a bug or a feature. I’d guess it is here for a reason, and maybe I’m late for noticing, but Rails 3 errors now provides an array for each attribute, whereas in Rails 2.3 it was just a string.  Here’s the output from two almost identical applications…

Loading development environment (Rails 2.3.8)
>> person = Person.new
=> #<Person id: nil, first_name: nil, last_name: nil, present: nil…
>> person.valid?
=> false
>> person.errors
=> #<ActiveRecord::Errors:0x1034d8f10 @errors=#<OrderedHash …
>> person.errors[:first_name]
=> “can’t be blank”

Loading development environment (Rails 3.0.0)
>> person = Person.new
=> #<Person id: nil, first_name: nil, last_name: nil, present: nil…
>> person.valid?
=> false
>> person.errors
=> {:first_name=>[“can’t be blank”]}
>> person.errors.class
=> ActiveModel::Errors
>> person.errors[:first_name]
=> [“can’t be blank”]

I didn’t see that in the release notes, but it failed my tests for ActiveRecord class. Someone else must have a list of these details, yes?