I'm usually pretty good about remembering how long I've had any given tab open, give or take a few days, so I usually don't ram into this too often. I can go a few weeks, in fact.
Last night, I prepared a comment, hit the submit button, then cringed at the sight of the familiar pa... wait.
Invalid form submission. You may have left the reply form open too long, or logged out since you opened the page. Please try posting again.
My comment was helpfully included in a box below the error message. The familiar, beloved error message from dear afuna's patch for Dreamwidth. It had been pulled upstream and quietly implemented.
I burst out in tears of sheer happiness. This. This is how it works.