Functional Programming - Is Set Theory Wrong?

Some of these things are completely impractical but to make a point out of this a lot of functional programmers who are into the sort of mathematical you know strengths of category theory type theory. They claim that you can mathematically prove that your function is correct and therefore your entire program is absolutely correct bug-free just whatever that means. too a little dangerous I mean some things that I've been told for example is well you can't really talk about set theory because set theory is wrong oh how is it wrong? "I think there are fields where this is actually very important to be able to prove that what was written is italines exactly with bold"