Yes, the thundering herd problem. While the site may be briefly less responsive, if the traffic is all to a single piece of content as long as one of them goes

Yes, the thundering herd problem. While the site may be briefly less responsive, if the traffic is all to a single piece of content as long as one of them goes through you'll end up with the content in cache and then the load immediately drops. The bigger problem is when your entire cache is cold (ex. memcached was restarted) and there's a ton of traffic to lots of different content. A single piece of content should not be that crippling, unless it's stupidly-slow to render and cache.