It's an impersonal verb construction. Si sa = one knows.
"Si" in this case is a formal third person pronoun. It's easier if you think of the sentence in English as "one never knows what can happen". It's third person in the sense that the conjugation of "potere" is the same as he/she/it (i.e. "puo"), but when you use the word "one" in English, it's a polite, formal way of discussing either yourself, the person you're talking to directly, or a third person not involved in the conversation. In this sense the word one can refer to many people.
Good point. "You" is an informal and colloquial way to do it, and can be misinterpreted by listeners as referring to them.
The grammatically correct usage is 'one' (singular verb) and 'they' (plural verb). "One never knows what may happen"; "they say that ..."; etc. @oktaya above says "one" is accepted here, and I've had both accepted elsewhere.
To the objection that it sounds formal or posh - well, it's supposed to! It's the si impersonale for goodness sake! Even so, it often sounds better than the passive voice.
I recall Montalbano and Livia flirting purely by tone of voice: Lui: Si può? Lei: Si può. Lui: Si può!, and off they go to bed. That doesn't translate well into either "you" or the passive.
Thomas - its a bit of a generalised statement, not necessarily addressed at only one person. In fact, its almost introspective. Its one of those idiom things, so we have 'sa' (for either he she or it). There is much discussion on the use of 'One' instead of 'You', here. Whilst it might sound a tad formal and not what everyone would choose to use, 'One' gets you over the hurdle safely and with the right verb conjugation!