Intel Core-2 processor (refurbished), Dell box, 2GiB RAM. I had all the everyday applications placed on the bottom 'panel.'  Then I installed several new packages using Aptitude.  Afterward. about half of the icons on the panel had disappeared (or become invisible).  The launchers are still there, and work; only the display is wrong. 
kubuntu 16.04: my panel icons keep disappearing after an upgrade
kde;kubuntu;plasma