Adding a validation to an ActiveRecord model over live data can cause sudden inexplicable brain-shattering headache. Why?
If the new validation causes existing rows to be invalid, any attempt to update attributes on the object without correcting the offending attribute will fail. This will happen, even if an html form that fronts the model doesn't provide an opportunity for your user to modify the attribute. And, boy, that can disorient your users. Any part of your application touching that row will become unwritable.
ActiveRecord's tight policy makes you think hard about what you're doing, which might be a good thing. It's hard to see alternatives that maintain ActiveRecord's simplicity. If your application never modifies rows outside of user-generated CRUD and if each model object corresponds to a single form, then you should be okay. But plenty of good application designs cannot meet both those criteria.
The best alternative behavior I could think of would be a "partial" validation. That is, a validation check only occurred on attributes that had been modified on the object, the "dirty" attributes, so to speak, letting other invalid attributes flyby. Eventually, an invalid row is going to kill you. After all, there is a reason it is invalid.
And while I prefer a noisy failure to a quiet failure, I'd rather not my application lockup and my users bear the burden of my oversight. I can even imagine a validations mechanism that strictly enforced partial validation, but performed full validation, alerting the developers in some loud cantankerous way if full validation failed but partial did not.
Depending on the size of your database, it might be a good idea to database validity during a migration. You could iterate through all rows, calling Active Record#valid? on each, and raise an exception failing on a validation failure. It could report or enact some other policy. Consider adding a migration for each such validation. A validation that invalidates existing rows is really no different than a database migration. After all, in olden times, much of the constraints expressed in validations were expressed in the database schema itself.