Friends disappearing on sidebar?

[Undead Tyrant]

I logged into Facebook a while ago and noticed that on the sidebar, a couple of my friends have disappeared. Their account wasn't deleted because their account is still there, nor did they unfriend me because I still have them on my Friends List. I searched them in the search box under the sidebar and they did show up, but they're not on the actual sidebar. I logged out and logged back in and they're still not there. Is it some type of glitch that will fix itself, or is it something else?
I looked at the advanced settings and I never messed with it. So hopefully it's a glitch that will fix itself.