bad.rb

irb(main):001:0> a = { :x => true }; b = { :x => false }; p a.deep_merge!(b, {:merge_debug => true})
Source class: Hash :: Dest class: Hash
Hashes: {:x=>false} :: {:x=>true}
 looping: :x => false :: {:x=>true}
 ==>merging: :x => false :: true
Returning {:x=>true}
{:x=>true}
=> nil

good.rb

irb(main):001:0> a = { :x => true }; b = { :x => false }; p a.deep_merge!(b, {:merge_debug => true})
Source class: Hash :: Dest class: Hash
Hashes: {:x=>false} :: {:x=>true}
 looping: :x => false :: {:x=>true}
 ==>merging: :x => false :: true
  Source class: FalseClass :: Dest class: TrueClass
  Others: false :: true
  Returning false
Returning {:x=>false}
{:x=>false}
=> nil