This was a strange one. After Site5 upgraded Rails to version 2.3.2, I began seeing the following error when executing a previously working RJS file:

ActionView::TemplateError (undefined method `[]‘ for #<Enumerable::Enumerator:0xb75bd33c>) on line #2 of app/views/store/pp_checkout.rjs:
1: page['pp_invoice'].value = @result
2: page['pp_checkout_form'].submit
3: page.replace_html “breadcrumb_trail”, “Please wait … transmitting data to PayPal for final checkout.”
4: page.replace_html “work_area”, :partial => ‘shared/paypal_processing’

Luckily, I found this post >>><<< which explained that a change to a method in the String class introduced in the rails 2.0.2 resulted in a missing method error being thrown by some previously working code. This error seems to be triggered by using the page['someElement'] or page[:someSymbol] syntax in RJS files.

Following the advice in the github WIKI post and then restarting the server immediately fixed this issue.