Ruby hash bug

playing with irb

1.9.3p0 :001 > h = Hash.new([])
 => {} 
1.9.3p0 :002 > h[:a] << 1
 => [1] 
1.9.3p0 :003 > h
 => {} 
1.9.3p0 :004 > h[:a]
 => [1] 
1.9.3p0 :005 > h[:b] = [2]
 => [2] 
1.9.3p0 :006 > h
 => {:b=>[2]} 
1.9.3p0 :007 > h[:a]
 => [1] 
1.9.3p0 :008 > h[:c] ||= [5]
 => [1] 
1.9.3p0 :009 > h
 => {:b=>[2]} 
1.9.3p0 :010 > h[:c]
 => [1] 
1.9.3p0 :011 > h[:a]
 => [1] 
1.9.3p0 :012 > h[:b]
 => [2] 
1.9.3p0 :013 >