Stewart, I noticed the same thing on the dev server when picking up items. I'll see if I can replicate it consistently. I definitely had room in my pack.
I had this happen a couple of times, although I cannot link it to a specific event that I did. I also had an odd experience twice, in which the entire backpack froze up. I couldn't use anything, look at the information for anything or access it in any way, although it still appeared visible.
This was slightly different to the bug described here (which I also got), in which I was still able to use the items that weren't failing to appear.
bluto: I'm unable to reproduce this, but I'm going to file a bug internally anyway to try to track it if it comes up again. There's a good chance that whatever was going on got fixed as part of other efforts or another bug fix, so hopefully this should be resolved. If it crops up again, please let us know either by updating this thread or posting a new one.