Sorry for the confusing comment -- I updated it. I was saying that it is strange for the linked post to talk about soundness being important when it's trivial to find an unsound Flow program. (This particular program is properly rejected by TypeScript but TypeScript is also unsound so there are plenty of other invalid programs that it accepts.)

I think it's a lot harder to get past TypeScript's type checker. It's probably possible, but I can't think of a way to do it off the top of my head without casting to any. It's a lot easier to show correctly executing javascript programs that would be rejected by the type checker.

Here's one that TypeScript gets wrong. It's kind of a canonical example of a program that is difficult to get right in a type system. (It's how I found the Flow example linked upthread.)

    let x: number[] = [];
    let y: Object[] = x;
    y.push('a');
    let bad = x.pop();
    // bad has type number but is actually a string.