[Mithaldu]

Specifically the mistake Javascript is making here is one designed into the language itself:

When there are no semicolons between statements, it will automatically and silently insert them. Without this behavior his mistake would've been a compile-time error and easily caught. (Of course, this leads to the bigger bug that sometimes Javascript will insert them in the wrong place.)

He doesn't know exactly what the bug in Javascript is and how to point it out accurately, but he is absolutely correct that there is a bug in Javascript's design.