1.rb

[1] pry(main)> def x
[1] pry(main)*   puts 'called x'  
[1] pry(main)*   callcc { |cc| return cc }  
[1] pry(main)* end    
=> nil
[2] pry(main)> def y
[2] pry(main)*   puts 'called y'  
[2] pry(main)*   x.call  
[2] pry(main)* ensure    
[2] pry(main)*   puts 'ensure block'    
[2] pry(main)* end    
=> nil
[3] pry(main)> y
called y
called x
ensure block
NoMethodError: undefined method `call' for nil:NilClass
from (pry):7:in `y'

2.rb

[4] pry(main)> def x
[4] pry(main)*   puts 'called x'
[4] pry(main)*   callcc { |cc| return cc }
[4] pry(main)* ensure  
[4] pry(main)*   puts 'ensure in x'
[4] pry(main)* end  
=> nil
[5] pry(main)> y
called y
called x
ensure in x
ensure in x
ensure block
NoMethodError: undefined method `call' for nil:NilClass
from (pry):7:in `y'