When you can login to a website perfectly fine but your client (the person paying you) can't for some reason. Still not sure if it's an isp issue (which I can't solve) or an site issue (which I should be able to solve) .

Update (11:15pm on 2017-06-19): Client desktop doesn't load the site after flushing dns (and trying different browser) . But the site loads fine on his phone so tomorrow I will be playing IT tech support (trying to figure out why client can't access the site today when he was able to access it yesterday). I love and hate what I have decided to do for a living.

Easiest way is just TeamViewer so you don't have to coordinate with end users