It's happened quite a few times after that. Consider the merchants who have been known to use fewer than the standard # of confirmations and what it takes to reverse or reuse those charges. For example, here is a blog post on one way to do this: https://blog.acolyer.org/2015/10/01/misbehavior-in-bitcoin-a.... If you want to search you will find that there are scripts to automate this type of action by constructing bitcoin transactions.

Also, consider ghash.io or the odd OKPAY double spends.