Handgard end cap, gas block and barrel nut, and maybe the handgaurd itself if it is a one piece design. Pick a break with a smaller od than the barrel (and allow some room for the weld) and you should be ok I think.
I had never really thought of this, but it is a good point.
This would be a problem if say you wanted to switch barrels and reused some of the components, or if you wanted to install a free float handguard.