I noticed that the site header was suddenly showing a series of vertical bars. These appeared to be where the header image repeated.
I opened the site in chrome dev tools and was able to remove the bars by disabling the svg background image.
The relevant CSSS is:
background-image: url("img/header-bg.png?v=4270316d9fec"); background-image: url("img/header-bg.svg?v=6d7dc0e93333"),none;
After chatting with @undo and @jon_ericson it appears the issue is only present at some zoom levels.
Jon said he would ask a designer to look into the issue.
Chrome 52 on Win10