The documentation states:-

"Execute the given block for a particular scope on the page. Within will find the first element matching the given selector and execute the block scoped to that element:"

However, if the selector returns more than one match, it seems to trigger an error rather than simply use the first matching element. According to the discussion here

https://groups.google.com/forum/#!searchin/ruby-capybara/ambiguous/ruby-capybara/z3Oh2jGR46U/zxZURjA-IjQJ

it looks like the observed behavior is the way it's supposed to work but in that case, I think the documentation is misleading.