It only happens when there are TWO pages of posts, with the first unread post on the first page, and subsequent posts spilling onto the second page. When you click on the thread header and the first page loads, the website automatically marks everything on the first page, including unread posts at the bottom, as "read". Clicking on the "first unread post" button takes you to first post on the second page, even if you haven't scrolled down to read the unread posts at the bottom of the first page.
I've gotten in the habit of looking at the number of total posts when I see only two pages under a thread. In the low to mid 20's, I will scroll down to see if there are unread threads (the blue "unread" marker will still be on them), then click on the "first unread" button to go to the next page. It's a nuisance for lazy pilots like me
Didn't see where anyone else had noted this, so I thought I would pass it along. I realized there were several times I had missed something in the conversation and had to go back to page 1 to catch up.
Thanks!

