Apologies if this is a dupe – I had a quick search and couldn’t see anything. I put it in UX because it’s not really a bug.
When replying to a topic, I’ve found that the posts I write aren’t automatically marked as read. This can happen if, for example, I start a reply, then jump back to the index or another topic to grab a link or a quote. It’s not hugely inconvenient – I just load the topic and let it sit for a moment or two – but I think it would be a fair assumption that I’ve read anything I’ve written.