I've had trouble articulating this, so hopefully the following code will make it pretty explanatory:

Code

(Variants also available https://gist.github.com/3519907 and https://gist.github.com/3519564)

class Hash
  def map_values
    map { |k, v| [k, yield(v)] }
  end
end

p data = {"x"=>[{"y"=>42}]}

p(data.map_values { |x| x.first })
p data

p data.map_values(&:first)
p data

Correct output (jruby-1.6.7.2 --1.8, jruby-1.7.0-preview2 --1.8, mri 1.8.7, mri 1.9.2-p290)

{"x"=>[{"y"=>42}]}
[["x", {"y"=>42}]]
{"x"=>[{"y"=>42}]}
[["x", {"y"=>42}]]
{"x"=>[{"y"=>42}]}

Incorrect output (jruby-1.6.7.2 --1.9, jruby-1.7.0-preview2 --1.9)

{"x"=>[{"y"=>42}]}
[["x", {"y"=>42}]]
{"x"=>[{"y"=>42}]}
[["x", ["y", 42]]]
{"x"=>[]}

In my specific case, not only does my method (contrived — simplified from a real world project) very much return the wrong thing, it mutates the original Hash, even though no mutative methods were used.