The status whether a page is watched or not is not shown correctly in the interface, as the page gets cached in the old version.

Steps to reproduce:

1. Visit a page on en.wikipedia you currently don't watch, clear your caches.
2. Click the star to watch it. The star turns blue (in Vector, in other skins the text of the watch tab changes), a message is shown that you now watch it, you can confirm this on https://en.wikipedia.org/wiki/Special:EditWatchlist.
3. Click the "Article" tab.

Expected result: The star is still blue.
Actual result: The page is sent as 304 Not Modified, a cached version is shown which looks like you are not watching the page. You can confirm on https://en.wikipedia.org/wiki/Special:EditWatchlist that you still watch the page.

4. Reload the page in a way that clears the cache. The watch star now turns blue as expected.
5. Click the star to unwatch the page. The star turns white.
6. Click the "Article" tab.

Expected result: The star is still white.
Actual result: Again, the page is sent as 304 Not Modified, the star is blue. Clearing the cache fixes the display, but shouldn't be necessary.