Dock icons disappear and/or go fuzzy Every now and then the icons in my Dock disappear completely after waking from sleep, except some of them go funny or fuzzy. See screenshot.

MacBook Pro Retina, 15", Mid 2014, i7, 16GB memory, often plugged into a Dell P2715Q 4K Display. El Capitan.

I've tried killall Dock in terminal to no avail, and rebooting doesn't do anything much.

Any thoughts on what's causing this, or how to force OS X to clear its cache of the icons or similar?
To clear the cache you need to delete the dock's plist.

Go to ~/Library/Preferences and delete com.apple.dock.plist.  Restart your Mac.  That should rebuild the icons.  I'm not sure why this is happening, maybe someone else here can help with that.