Ruby

This segfaults:

class C
  class << self
    attr_accessor :val
  end
end

GO = false
rs = 100.times.map do
  Ractor.new do
    Thread.pass until GO
    1000.times do
      val = C.val
      p val
    end
  end
end
ts = 100.times.map do |i|
  Thread.new do
    Thread.pass until GO
    1000.times do
      C.val = Object.new.freeze
    end
  end
end

GO = true
ts.each(&:join)
rs.each(&:take)

It looks like a silly example, but it can crash even if the main thread doesn't spawn any more threads. Getting/setting ivars on objects in parallel without VM lock isn't safe, so for non-frozen shareable objects (basically classes and modules), it's unsafe to access their instance variables even if the values are shareable.