Intermittent "Your browser session has expired and we are unable to complete your form submission" errors
Based on further investigation and discussion this seems to have been related to the WP Super Cache "Don’t cache pages with GET parameters. (?x=y at the end of a url)" setting being disabled. Once we got a bit of a spike in donors some people started seeing the error, but luckily a donor notified us so we caught it pretty quickly, and I was able to flip back to a Gravity Forms donation form as a stopgap while we did some troubleshooting. Live and learn.