gistfile1.txt

1.9.3p194 :001 > 1==1?"one":"not one"
 => "one" 
1.9.3p194 :002 > def one
1.9.3p194 :003?>   1
1.9.3p194 :004?>   end
 => nil 
1.9.3p194 :005 > 1==one?"one":"not one"
SyntaxError: (irb):5: syntax error, unexpected tSTRING_BEG, expecting keyword_do or '{' or '('
1==one?"one":"not one"
1.9.3p194 :006 > 1==one ? "one":"not one"
 => "one" 

1? is not a valid method name, while one? is?

I always thought method names had to begin with either a lowercase letter or underscore. http://rubylearning.com/satishtalim/ruby_names.html

I'd rewrite it as a (blah if foo) || bar to show it in a terser form. In which case yes, it would look to be because 1? isn't a method name, but one? could be.

("one" if 1==one?) || "not one" # => 
# ~> -:7:in `<main>': undefined method `one?' for main:Object (NoMethodError)

("one" if 1==one) || "not one" # => "one"