Has anybody heard of a situation where somebody didn't pass "viva exams"?

Unfortunately, yes. I have actually witnessed someone come out of their viva after failing. To be honest though, if you're likely to fail your supervisor should pick up on it early on and do something about it.

Do they get a chance to resit or what?

You can 'fail' a viva in several ways. If the thesis was OK, but you did very badly in the viva (unusual), you can resit the viva after more preparation. If the thesis was not OK you can be asked to do more experiments and resubmit another thesis, receive a Masters rather than PhD, or fail outright (no resubmission allowed). For the thesis not to be OK means that the supervisor does not know their stuff, or that the student has ignored their advice.
Examiners are actually reluctant to fail a thesis for fear of being involved in legal action!

I have not awarded the PhD on one occasion. The candidate ignored the advice of the supervisor, had not amended drafts as advised and reached the registration time limit. The thesis was rushed and fell far short of publishable standard (which is a requirement). As examiners, we recommended that a rewritten thesis was submitted within 18 months. 18 months are up and there is no sign of any rewritten thesis. The candidate has effectively failed herself. It's a sad tale and hopefully a rare one. Be guided by your supervisor and adhere strictly to the university's regulations.

I started my PhD in September last year. I've recently encountered some financial trouble and I can't afford to pay my tuition fees. Would I be able to have a year out to get the cash together?

It is unlikey you will be able to do this on a formal basis, but talk to your supervisor and see what they suggest.