In the hotel example, it's the lack of a modern, open, integrated reservation system. It's all 20+ year old systems patched together to try and work with one another, that don't necessarily have any idea what inventory at the hotel actually is unless it's part of a big organization like Marriott/Hilton/IHG/Wyndham/Choice/etc.

When a hotel doesn't have a 24/7 online system like the above companies, booking.com simply relies on the hotel to update their inventory on booking.com's system manually, and then faxes the hotel everytime a reservation comes through. The fax can fail, and so can the minimum wage employees who may or may not be tasked with updating booking.com's systems.

Bottom line is always confirm, and re-confirm when the consequences are great enough to warrant spending time re-confirming. And especially try to deal with the vendor directly, onsite especially. If you make a reservation at a hotel that doesn't have an app that shows your reservation in real time, then call and get the email address of someone who actually works on site, and then email them to confirm your reservation, and so you have something in writing from someone at the hotel.