I've seen some examples of this happen here in Australia with Transformers, but today I came across this site that had a category of "Toy Trolls"... for submissions of returned toys that had something else repacked in them, or something missing. (the site is on hiatus from the start of this year, but the archived postings are there)

Aside from people opening up the packaging in the store to steal items, Retailers have procedures in place (and one is a legal requirement in Australia) that makes this crime of returning repacks very easy to do, but I'm not going to go into detail of course.

Which is why it is always important to check the contents if you can see them, and check the seals on cards and packaging when buying from Retail. (they can exchange or refund, but do you want that extra hassle?)