Do you find yourself accidentally doing that often? A few of the things from the "Wat" video fall into that category. "Things no one would have ever thought to try, yet a dynamic language will fully allow". If you want to point out valid concerns with JS, the double-equal implicit coercion is a far more frightening "gotcha".
Not lexically, no, but dynamically, sure, all the time. JS lets me receive an arbitrary value in a variable `foo`, and "add" it to another arbitrary value in a variable `bar`, without having any opportunity to specify what semantics I mean by "add." Fixing the semantics of the operation is the whole point of specifying it!
Personally, I favor languages that go "almost too far" in the opposite direction, e.g. Elixir. In Elixir, operator[] (a.k.a. the Access protocol) is generic across several types, but the semantics are very strictly constrained; not only does the access have to "mean" getting some kind of element from a container, but it's also only defined where it has a fixed O(1) time-complexity.
This sort of design being conventional in Elixir, means that you can usually predict, just from the lexical environment (a.k.a. "the code on the screen") what the runtime behavior will be of said code—which includes being able to "read off" a time/space complexity for the code—without needing to check how each ADT involved has implemented its generics, let alone having to check for implicit type coercions.
Few people do that outside of code obfuscation contests, but the point is JS has heaps of gotchas like that, each waiting to bite you. How about:
x = 5;
(no var declaration). Extremely easy to do by accident. Lots of these will be left for backwards compatibility. In JS errors often go silent and I developed a strong dislike for any language that does that, including Vimscript and Lua (and I do like Vim).
I'd agree that global declaration is far more egregious. A sibling comment recommends a linter, and I'd probably agree. It's sad that it was ever allowed as one that is copying and pasting code to another location can easily cause this case to happen.
Eh, I actually like JS but I have definitely forgotten to add a "var" or "let" a few times, or sometimes I will do it correctly, get a bit overzealous with cutting, and accidentally remove the initial var declaration.
I'm not saying that it breaks the language, but it definitely is irritating, especially if you're not using a linter.
Whilst I do find it irritating, I don't mind about the scope default so much, but I've certainly run into it (forgetting to specify) a lot.
Mostly because JS is not the only language that I'll be working with at a time, and if I'm dancing between three or four and they have similar syntax, then mistakes creep in.
