Carlton City Hotel Singapore, I arrived there in the middle of the night with a printout from booking.com of my (already-paid-for) reservation.

The staff said they don't have my reservation, told them to check booking.com, they replied they do not have access to booking.com, a third party manages that for them which was unavailable at night. In that moment I realized I was putting way too much trust in that website. I was able to pay for a new reservation and later got the booking.com payment refunded. Luckily the hotel was not fully booked...