var a = 17, b = 13; c = 5;

(usually this will be across multiple lines)

...just overwrote c in a different scope. This kind of bug is common, idiomatic, baffling, and actually more likely among coders subscribing to javascript "best practices".

use strict? jshint / jslint your code?

This doesn't justify playing a guessing game and skipping semicolons just because you think you know all the rules about not using them.

jslint won't stop it if there's a variable (c in this case) in the outer scope -- it's perfectly fine code as far as jslint is concerned. And jslint encourages that declaration style (actually encourages is too weak a word).