"In nine out of ten cases it goes well."
Translation:I ni av ti tilfeller går det bra.
Here, it's more likely that "cases" just refers to instances or occurrences of some sort, which "tilfeller" covers well. Another option would be to phrase it as "Ni av ti ganger går det bra" instead ("ganger" = "times").
"Sak" has many applications, but won't always translate to "case". A court or legal case would be "en (retts)sak", and a police case could be referred to as "en (politi)sak". "En sak" can also be something you want to discuss with someone; a matter. It can also translate to "a thing" in the physical sense.
So, if "cases" is indeed referring to legal or police cases in the sentence above, then "saker" is a good translation. However, in most other cases it would earn you a confused look.
"tilfelle" er ikke en skrivefeil, men en form av pluralis: https://ordbok.uib.no/perl/ordbok.cgi?OPP=tilfelle