"Please manually invalidate…" what does this mean

Yes – you just need to clear the W3TC cache.

FYI, I also get the same message when viewing “Settings > Disable Comments”. The odd thing is that I don’t have a caching plugin installed on my site. It is a test site, so I may have installed & deleted a caching plugin in the past. I saw a ../wp-content/cache folder, but it was empty and had a 2-year-old date, so I deleted it and checked “Settings > Disable Comments” again. The message was still there.

I’m running WP 3.6 and a child theme of WooThemes’ Canvas (the current v5.3.0, and the current WooFramework v5.5.5). I switched to Twenty Thirteen to see if it was a theme issue, but the message was still there. I also do have a number of other plugins installed. So far I haven’t seen any ill effects, so this isn’t any emergency. ?? Cheers!

@thenightrider, the plugin checks to see whether you have defined the WP_CACHE constant in your config file – this tells WordPress that you’re using a caching plugin.

Maybe you had done this when you installed a caching plugin in the past, and then forgotten to remove it. Have a look in your wp-config.php. If you remove that line the message should disappear.

@solarissmoke, thanks, you are right! WP_CACHE was set to ‘true’ in my config file. Removing it removed the message. I searched my log and, sure enough, I had previously installed WP Super Cache. I guess it didn’t clean up after itself when I deleted it.

I really appreciate your answering so quickly, especially since I could have looked at the code and seen the ‘if( WP_CACHE ) ‘ starting at line 292. ?? It’s good to have this answered here, though, for the many people who don’t read code. Cheers!