how.. do people accidentally get pregnant?

Not to be a smart ass.. but i’m genuinely curious. I know some women that are incredibly smart but somehow still tell stories about accidental pregnancies.

personally, i am terrified, every time me & my boyfriend do anything, i’m always checking & making sure everything’s right & triple checking the condom.

i just don’t understand how practicing safe sex leads to pregnancy? i hear stories about women not wanting their kid because they were an accident or someone had a baby & somehow already is expecting another..

Has it happened to you? How did it happen? Was it out of ignorance or genuine accident??