I've been having this problem for quite a while now, never bothered me enough to investigate further, I really want to get this resolved now though...
It occasionally happens that I have String lists in my UI (like an addons option subcategory or Acherons death times - happens in all clickable lists) that are empty. I can choose the corresponding menu point, all works, just the strings won't show up. Since this isn't reproducible and not fixed to a single addon I am really at a point where I can't reproduce where this is coming from. Has anybody seen similar behaviour in his UI?
I've seen this happen in Acheron where the left scrollframe showing each death is only filled from about half way down with the top half missing their strings. I've also seen it in the Interface -> Addons list where only the bottom half of the scroll list are showing.
I've got no idea what causes it to happen either, but a reloadui fixes it.