The same thing almost happened to my mother last October in Florida.
The day before I planned to go to Magic Kingdom, she decided to join me on impulse. She bought her ticket, and I tried to talk her through the reservation process over the phone. She was sure she did it correctly, and that was that.
Next day: apparently my Mom didn't do it correctly because according to their system she didn't have a park reservation. They weren't going to let her into the park or even onto the ferry boat to get from the TTC to the actual park. The only reason they budged was because she was able to convince them she had a dinner reservation in the park (thankfully they didn't check this-it was a reservation for two but it didn't specifically have her name on it). Otherwise they wouldn't have let her in and she would have thrown $150 down the toilet (since our schedule wasn't flexible and the WDW 1 day ticket expiration window is now literally four days).
Which begs several questions:
1. Why doesn't the system require you to make a reservation before you are allowed to buy the ticket, ESPECIALLY if it's a one day ticket, ESPECIALLY if it's literally for the next day?
2. Why are they so stringent on enforcing this, ESPECIALLY if the day before there was clearly no danger of running out of park reservations?
3. How can a company still smarting about its superior customer service continue to enforce such a policy so strictly (and in direct contradiction to how they tend to enforce their other rules) and talk about how awesome their service is with a straight face?
I'm sure it's helping Disney from a cost saving and staffing perspective, and that's why it's in place. But do they really think that choosing park reservations as the one hill they will die on is in their best interest as a business? Do they really think that these people who are turned away after they spent the money will ever go back to Disney after that ordeal, with those people's takeaway being "lol whoopsie I guess I'll do better next time" and not "I will never give this business money again"? It beggars belief.