There's a prototype and an instances that inherit from prototype. Prototype has defined given property with getter, which on first call defines static value for that property on instances with which it is called. It happens that after property is defined with static value, still underlying getter is called. It's reproducible always for given cases. I didn't manage to come up with simplified use case. Problem is well described with that gist: https://gist.github.com/medikoo/9268228 but unfortunately not reproducible with it. To reproduce it, follow below steps: 1. Get into http://ldz-test2.medyk.org/ it's dev version of application I'm working on with not minified scripts so you can more easily track what's going in 2. Open Web Inspector with console visible 3. Login (blue "INGRESE" button in top right corner) with email: mariusz@medikoo.com password: 11111q 4. After page is loaded click on tab: "2 Ingrese sus datos" page would not load fully due to an error. See output in console, I've added special console.error call in getter which should not be called, see it's stack trace pointing to lines in unminified JavaScript code There are many other scenarios in the application when that error occurs (always error is logged) but not always they result in crash. This site works perfectly fine in Firefox and Chrome.