A Steam user since HL2 here, and I got locked out from my Steam account 9 months ago for using a particular mobile phone browser for Steam purchase (Steam Mobile wasn't available at that time). When I asked the support when I could get my account back, they said the lock was permanent; I realized I just lost 300 of my games. It was dreadful. With a help of my friend I found Steam staffs' forum account, and literary had to beg to get my account back.
During that time I read the posts of guys who also locked out of their account permanently. There was a guy who lost his account due to someone gift him a game and its payment had an issue.