1. Open Mattermost in two browsers as two separate users. Make sure that caching is enabled (and possibly close the browser's dev tools)
2. Have both users join the same channel
3. Have the first user make a post in the channel (clears the etag cache), switch channels away, switch channels back (triggers an empty getPostsSince to cache 0, but doesn't return an etag), and then refresh the page (calls getPosts to save the etag in the browser).
4. Have the second user repeat the same process of making a post, switching away, switching back, and then refreshing the page.
5. Have the first user refresh the page. The first user should be missing the second user's post, although they may be missing more if a 0 etag was previously cached.
Observed: You may see an older version of the posts in the channel
Expected: You see the latest version of the posts in the channel
Happens on community-daily, community