From time to time, Happo's infrastructure is updated to run newer versions of browsers. When that happens, there's a chance you see a large number of diffs even though you didn't introduce any changes in your application.
Example of a diff caused by an updated Safari worker. The newer version of Safari rendered the font slightly different from the previous version causing a diff to appear on the component's label.
When this happens, you will most likely see an alert at the top of the report outlining the changes made to Happo infrastructure that might have caused the diffs. Annoying as this might be, it's usually safe to simply accept the report and move on (once you're confident that the changes are solely caused by the browser update).
As a result of some browser updates, diffs can appear that aren't showing any pixel differences at all. When this happens, the most probable cause is that the underlying browser driver has changed how it creates screenshots, causing different bytes in the captured PNG image than it used to do. This confuses Happo's diff resolution, since it relies on computed hashes of the PNG images (known as "bitmap hashing"). If the hashes are different, Happo assumes the pixels are different.