[Rails] Rspec unit test fails sometimes (validation race condition)

I have code similar to this:

class Post
has_many :comments
end

class Comment
belongs_to :post
validate :must_not_exceed_ten

def must_not_exceed_ten
errors.add_to_base('Too many comments') unless post.comments.length
< 10
end
end

and in the specs (Rspec + machinist):

post = Post.make
9.times { Comment.make(:post => post) }
comment = Comment.new(:post => post)
comment.save.should be_false

The weird thing about it is that it fails sometimes! Especially if I run
the whole test suite with rake command (the probability is higher). When
I check in the console post indeed has 11 comments. Can somebody give me
a reasonable explanation why this is happening? Is this some sort of a
race condition that I'm missing?
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