How do you not realize that the obvious answer to your question (in the OP) is yes? There are quite a lot of people that have lost Bitcoin through various events (e.g. theft, sending to wrong addresses, hardware failure, no wallet backups and ect.). However, fortunately I can say that I'm not one of them. I've successfully avoided a lot of problematic events such as (Mt.Gox, Inputs.io, Cryptsy) and have properly managed to keep my wallets safe and secure.
Here is a story where someone lost 7500 bitcoins.
A story.