Well, that's after I changed this:

    begin
      s1, s2 = two_sum(arr.dup, target)
    rescue NameError
      s1, s2 = twoSum(arr.dup, target)
    end

For this:

    if defined?(two_sum)
      s1, s2 = two_sum(arr.dup, target)
    else
      s1, s2 = twoSum(arr.dup, target)
    end 

Any idea why the first one gave that error instead of the out of bounds? I mean, hiding the real problem.

main.rb:14:in `block (2 levels) in two_sum': undefined method `+' for nil:NilClass (NoMethodError)
	from main.rb:12:in `loop'
	from main.rb:12:in `block in two_sum'
	from main.rb:11:in `loop'
	from main.rb:11:in `two_sum'

As it says, one of the arguments of the + inside the inner loop is nil, which means the index is out of bounds.

I saw it, but still I have no idea why that happens, sorry. Maybe someone who's more fluent in Ruby could find out why.

I'm gonna explain why that happens (I hope), but that part of the code should run only if your function isn't called two_sum.

    begin
      s1, s2 = two_sum(arr.dup, target)
    rescue NameError
      s1, s2 = twoSum(arr.dup, target)
    end

And it's there for compatibility reasons. The Ruby translation used camelCase instead of snake_case at first and then that was changed. No idea why it would pass some tests and fail randomly. It doesn't happen with my solution.

It's not that test you are failing but the previous one. Print the input at the beginning of your code.