The worst problem with almost all popular programming languages are null and shitty error handling, such that, when you call foo() where foo is String foo(){ blabla } you can get a String, null OR an exception/hidden error response, but most compilers happily let you treat it as if it only ever returns a String. (What!!!?)

I hope some day null is no longer a thing, and that Functional Programming types like Option, Either, Try etc in the native libraries is the new default. (BTW, there's really nothing "Functional Programming" about them, they're just objects like any other in "Object Oriented" languages)