Okay, this is kind of weird and I thought I was just forgetting what I had already read... but on the pets forum just now I KNOW that the system skipped a couple of posts after this one, and then took me to a later post ...
http://garden.org/thread/view_...
if that makes any sense.
In other words, I made a post, then later went back to the thread and when I clicked on "first unread" it took me several posts past the last one that I had actually seen.