It can be anything, really. Look at the reviews for items. Sometimes you'll notice an item has great, 5-star reviews and about 10% are terrible 1-star reviews. If you read through those 1-star reviews you might notice that sometimes there's a trend of the reviewer discovering they received a fake.

How widespread is the problem in reality? I have no idea. But given that I can simply shop elsewhere, get a similar experience, and feel more certain that I'm not receiving a counterfeit, I'll do just that.

I'm a consumer, not an analyst. If I think that I might receive a counterfeit from Amazon I'll just shop elsewhere, I have bigger things to worry about.