Reason being that in general text we don't read based on individual letters, we recognize "shapes" of words.
That's because it deosn’t mttaer in waht oredr the ltteers in a wrod are, the olny iprmoetnt tihng is taht the frist and lsat ltteer be at the rghit pclae. The rset can be a toatl mses and you can sitll raed it wouthit porbelm. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
Your Passcode is: 6234024
It's pretty noticeable with the monospace font that used on my system because O is more like a circle and 0 is thinner. There is no slash, and it is fortunate because it could be mistaken with Ø otherwise.
Another option, like on my system, is that zeros have a dot in the middle.
Is it actually a zero or am I missing something?
In Arial, they're indistinguishable. In Roboto, you can see the difference if they're right next to each other (l is longer than I). Segoe UI has different variants but the one I've found on the MS web site has very similar or identical lI's, just like San Francsico.
I know, fixed width fonts would fix that, but not everything that should will be in those. And especially on a programming website, that matters.