It's error prone, just like Zed says. Add braces, don't be lazy, it makes the code flow easier on the eye.
IMO, In JS, it's painfully dangerous to be "too" clever as well.
Sometimes I like to think about it like so: Will someone else "with less skill than me" be able to follow this code after me?
Then again, being human I sometimes fail to think this way :(.
Complaining that this isn't easy enough is missing the point.
Arguing that the language shouldn't allow such constructs is again outside the scope of the argument. K&R designed the language, so presumably they agree with the design.