Ideally, yes. But humans make mistakes, computers glitch. LH refused to let me board in FRA as they said that I didn't fly BOS-FRA, despite having a boarding pass and being in FRA.... I certainly didn't teleport.

Also crew can make mistakes or just be bad at math. Had an AA flight where we ended up having to count off because the FA messed it up 4 times.

And of course, passengers don't listen. Count is done and someone pops out of the lavatory right at the end. Another time, a flight I was on was delayed as they were missing a kid - kid was onboard, the brilliant parents had allowed their kid to crawl underneath the seats and didn't say anything.

It's pretty rare overall. But plenty of opportunities for someone or something to make a mistake.