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.
I'm not saying that it breaks the language, but it definitely is irritating, especially if you're not using a linter.