This question already has an answer here:
I am planning a trip to Hong Kong in June. When I tried to book my return flight (a direct flight from HKG to SFO) on Singapore Airlines's website, it requires that I enter my passport information (number, expiration date, etc.). My passport is currently being renewed and I don't have this information yet, but I'm hoping to book the flight now while the fares are reasonably low. My friend, who booked the same flight, seems to be able to change his passport information for his booking without issue.
My question is: Can I just put in made-up information for now, and then enter in the real information when I get my new passport? What can go wrong?