I cancelled a reservation I made with them for a hotel in Iceland and somehow, it never went through their system. I didn't get a confirmation email, and in hindsight, I wish I had contacted them about it, but it didn't worry me at the time because my booking seemed to be removed from my account. (I had booked 3 other hotels with them and those are the only ones that showed up on my personal account page. If my other booking had not been cancelled, it should have showed up on that page.)
Now, I'm being charged by the hotel with a no-show fee of the entire amount it would have cost to stay there. Booking.com refuses to take any responsibility for it even though I've sent them a screen shot of my account page which clearly shows this reservation doesn't exist. The kicker is that the hotel I actually did stay at was also booked through booking.com and for the same dates! And even that doesn't prove anything to them. I had to email several times before I even got anything other than a stock answer from them about the situation. Not sure if anyone was even reading the emails.