Users of Object#freeze should know that the behaviour of this method has been changing over the Ruby versions. Let’s compare it.

Ruby 1.8.7p370

nil.frozen? #=> false
nil.freeze
nil.frozen? #=> false

69.frozen? #=> false
69.freeze
69.frozen? #=> false

6.9.frozen? #=> false
6.9.freeze
6.9.frozen? #=> false

:snow.frozen? #=> false
:snow.freeze
:snow.frozen? #=> false

Ruby 1.9.3p392

nil.frozen? #=> false
nil.freeze  #=> nil
nil.frozen? #=> true

69.frozen? #=> false
69.freeze
69.frozen? #=> true

6.9.frozen? #=> false
6.9.freeze
6.9.frozen? #=> false

:snow.frozen? #=> false
:snow.freeze
:snow.frozen? #=> true

Ruby 2.0.0p0

nil.frozen? #=> false
nil.freeze  #=> nil
nil.frozen? #=> true

69.frozen?  #=> true
6.9.frozen? #=> true

:snow.frozen? #=> false
:snow.freeze
:snow.frozen? #=> true

Ruby 2.1.2p95

nil.frozen? #=> false
nil.freeze  #=> nil
nil.frozen? #=> true

69.frozen?  #=> true
6.9.frozen? #=> true

:snow.frozen? #=> true

@charliesome explains the difference.

The difference between 1.9.3 and 2.0.0 is that on 64 bit platforms 2.0.0 uses flonums, which means Floats are immediate values just like nil, true, false, Fixnums, Symbols, etc.