I am using the CSS Upgrade and am adding it to the Coraline Theme.
Today, one of my users commented that my Blog was not showing the modified theme - but the usual Coraline theme. Everything looked fine for me and my other users. Shutting down the browser and restarting didn't help (although I must admit I didn't ask them to "do the usual trick").
As an experiment I removed a big chunk of my CSS and they reported that things were back to normal.
After this, I viewed one of my blog pages and it was briefly displayed with the usual Coraline theme, but them automatically re-dipsplayed with the modified theme.
All users are running Firefox (myself included).
The blog I need help with is brickiesgazette.wordpress.com.