So we released to production today (Friday), not my decision.
All pages work fine expect for the one page which I added a new feature.
It worked fine in Chrome and Edge. But after release a customer who requested the feature said it doesn't work for him. Screenshot showed he was using IE.
Horror time.. it was evident that it has to be the changes to the JavaScript I did, but why does the whole page doesn't work.

So I started debugging. Nothing works on that page in IE11, it doesn't even load the fucking script file. Then I dared to change mode to IE10, it actually gave me an error in my script file. The bad IE has actually picked a mistake that other browsers didn't.

So, the mistake is fun part too.
I had the following jQuery (or Jake Weary) call
$.getJSON(
'/url',
{
argA: a, argB, b, argC:c
},
function (){
// did something
}
);

In second argument, I accidentally typed comma instead of colon. Chrome and Edge ran the script perfectly passing all the arguments.IE 11 failed to load script without giving any error and only IE 10 gave an error of expecting a colon.
I do not know which browser to blame.

PS I didn't try in Firefox, safari, etc.