Not just in Javascript. Since blocks define a scope, visibility and unexpected lifetime issues may occur in other curly-braces languages like C and C++ just as well.

Using blocks like that is something you should be very careful about - might have some surprising effects especially for novice programmers.

The blocks here act exactly the same as any other blocks, like the ones you write for loops and ifs etc. So i wouldn't exactly call the consequences "unexpected".

I can't recall right now but I remember having errors that would not happen inside a normal for loop block. And I was using let/const not var.