Random clients will have this issue when they arrive on the log in page, they are presented with a stylesheet that has almost no CSS. When looking at the console they style sheet that is loading is something like the following:
 

http://URLShortname.OurURL.com/App_Themes/Default/CSS/StyleSheet.ashx?guid=AY8GAHHAABAB&fileMask=Common/;FontAwesome;Mail/;HyperControls/&rtl=false&colorScheme=fa5bd08b00c14e128664dbcee1d86a53

The only css in the style sheet is:

.stripped-list{list-style:none;margin:0}.text-align-center{text-align:center}.float-left{float:left}.float-right{float:right}.text-align-left{text-align:left}.text-align-right{text-align:right}.float-left{float:left}.float-right{float:right}.text-align-left{text-align:left}.text-align-right{text-align:right}.float-left{float:right}.float-right{float:left}.text-align-left{text-align:right}.text-align-right{text-align:left}.stripped-list{list-style:none;margin:0}.text-align-center{text-align:center}

The only way around this currently is to have our clients perform a hard refresh. This isn't ideal. Also this was happening before 15.7, it's just more common now. Is there a reason why the stylesheets aren't loading? Is this a known bug?