Odd issue that has taken me forever to figure out what's causing it. Here is a little outline what happens.
p = Product.first #valid product
u = User.first #valid user
pu = Purchaser.from_user(u) #generates a valid purchaser object
#=> #<Purchaser _id: 4c49eba6b57f988f32000001, name: "Timmy Zuza", created_at: nil, quantity: 1, updated_at: nil, user_id: "4c43cc48b57f983010000001">
p.purchasers << pu
#=> [#<Purchaser _id: 4c49eba6b57f988f32000001, name: "Timmy Zuza", created_at: nil, quantity: 1, updated_at: nil, user_id: "4c43cc48b57f983010000001">]
p.save
src/tcmalloc.cc:353] Attempt to free invalid pointer: 0x2253d0

The error actually kills the ruby process
Here is a gist of my setup and more details: http://gist.github.com/487924
(I simplified the product.rb quite a bit)

The way to avoid this issue is to take out "embedded_in :product, :inverse_of => :purchasers" from purchaser file. I'm hoping this will help you track down the issue.