I went to http://en.wikipedia.beta.wmflabs.org/wiki/Talk:Flow_QA in a new anonymous session (incognito in Chrome, private window in Firefox).
The Flow board defaulted to Newest topics sort, but according to it themost recent topic was two weeks ago, starting with
Hidden Topic0.11943746546696965 browsertest edit This topic was hidden by Selenium user Active 2 weeks ago
There have been literally hundreds of newer topics since then. It's good to see caching working, but ... :)
I'm not sure if this happens in production, if so then this is high priority.
To ease debugging, could Flow put something like
/* content from BagOStuff cache timestamp=Xxx */
in its output when it shows cached content?