Ian bought a Kindle and some Kindle ebooks from Amazon. He also bought some real-world stuff from them, some of which he returned. Amazon decided that he'd returned too many things, so they suspended his Amazon account, which meant that he could no longer buy any Kindle books, and any Kindle subscriptions he's paid for stop working.
After some phone calls, Amazon granted him a one-time exception and lit his account up again.
Leaving aside losing your subscriptions, this would not be such a big deal if the Kindle had graceful ways of putting competitors' ebooks on your device. What's your experience getting non-Kindle books onto the Kindle?