a.rb

require 'set'
require 'monitor'

log = []
END {
  log << "end"
  puts log
}
m = Monitor.new
h = Set.new
f = ->(objid){
  begin
    m.synchronize do
      log << "delete #{objid.to_s(16)}"
      h.delete(objid)
    end
  rescue Exception => e
    log << e.to_s
  end
}
loop do
  a = Array.new(10000000)
  ObjectSpace.define_finalizer(a, &f)
  m.synchronize do
    if h.include?(a.object_id)
      raise "#{a.object_id.to_s(16)} already included!"
    end
    h << a.object_id
    log << "insert #{a.object_id.to_s(16)}"
  end
end

output.txt

YUKI.N@aincrad> ruby a.rb
insert b1394c
insert b27f78
can't be called from trap context
insert b27f00
can't be called from trap context
insert 919b64
can't be called from trap context
insert a21f34
can't be called from trap context
insert a21e6c
can't be called from trap context
insert b27eb0
insert b27dfc
can't be called from trap context
can't be called from trap context
insert b27d98
can't be called from trap context
insert 91977c
can't be called from trap context
insert a21fac
can't be called from trap context
insert a21e08
can't be called from trap context
end
a.rb:26:in `block (2 levels) in <main>': b27eb0 already included! (RuntimeError)
        from /usr/lib/ruby/2.2.0/monitor.rb:211:in `mon_synchronize'
        from a.rb:24:in `block in <main>'
        from a.rb:21:in `loop'
        from a.rb:21:in `<main>'