The file:

$ cat f.rb
o = Object.new
def o.method_missing(*)
  'abc'
end
p("" << o)

jruby 1.7.12 (1.9.3p392) 2014-04-15 643e292 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_51-b13 [darwin-x86_64]

$ ruby f.rb
TypeError: can't convert Object into String
  concat at org/jruby/RubyString.java:2601
  (root) at f.rb:5

ruby 1.9.3p545 (2014-02-24) [x86_64-darwin13.2.0]

$ ruby f.rb
"abc"

ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]

$ ruby f.rb
"abc"

If, however, I define respond_to?:

$ ruby -v
jruby 1.7.12 (1.9.3p392) 2014-04-15 643e292 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_51-b13 [darwin-x86_64]

$ cat f.rb
o = Object.new
def o.method_missing(*)
  'abc'
end
def o.respond_to?(*)
  true
end
p("" << o)

$ ruby f.rb
"abc"

Obviously, it's not a great idea to do this, but I think it reality it's pretty common, lots of people define method_missing without defining respond_to, and I'm pretty sure this is what's causing the currently released version of Pry to be unable to show output for whereami and show-source (pry/pry#1284)