I sometimes see addresses with negative balance.. like -0.05 BTC
I don't understand how it possible.
It's not possible and you don't see it. What you see, is a website with bugs. I assume you're talking about blockchain.info, I haven't seen this on any other block explorer.
Where can I read about this features?
I wouldn't bother about it, it's not a feature.
it's a bug of double-spend when making transaction.
It's important to note this is not a Bitcoin bug.