Those who can go in without a library card are required to show their library card, but most of the time no one cares. However, those who don't want to lend you books must register with a library card.

If you go in tightly, no one will doubt you. If you are really unlucky and asked to show your library card, you can say that you forgot to bring it. If you don't pick up this book, you'll just look around and maybe let you in.