If you reply to a topic and give it a subject other than the default, then on the main page it will say for that forum that the Last post was in the (whatever subject name you gave) topic, even though that's not the name of the actual topic. It seems confusing to me because someone might think a topic exists and it doesn't, or they'd click the forum and then struggle to find what they were looking for. I'm not aware of people routinely changing the subject of their posts (I always forget that field is there), so I'd never expect to see a topic name change for no reason. Is this behavior intentional?
