The problems were caused by a combination of abnormal network latency and cache frontend server configuration. There also was another problem that caused occasional "Oops, there has been an error" -messages when no error actually occured. This was also caused by an overly optimistic default setting in the version of Varnish Cache we were using. The setting has now been fixed along with an upgrade of Varnish.
We are deeply sorry about the inconvenience the problems have caused, and hope that they are now behind us.
Should you encounter problems accessing or using our site, please don't hesitate to contact me directly via email at firstname.lastname@afterdawn.com or via Twitter at @ketola or @afterdawn.
Reference:
https://www.varnish-cache.org/trac/ticket/1012
Written by: Jari Ketola @ 7 Nov 2011 16:37