My app icons in my dock, in the Applications folder, etc. periodically turn into the system default icon. 
It’s not all applications, and it’s not even the same applications every time. It seems intermittent.
I Googled, and I’m seeing a number of ridiculous solutions - i.e. “restore from your last known good backup”.
I’m sure there’s some sort of cache or something that’s goofed up - anybody know how to reset whatever is wrong?
Have you tried turning off the computer and back on again? That usually fixes it for me.
Killall Dock doesn’t do it?
killall Dock doesn’t fix it.
When are they changing (when you wake the computer up from sleep, when you restart, randomly during use, …)?
You could try:
Menu > Go (press option) > Library > Preferences
remove “com.apple.dock.extra.plist” and “com.apple.dock.plist” then restart.
Or:
“defaults delete com.apple.dock” in terminal and then killall dock.
Did this help?
If not, you can try to have Onyx purge your caches to see whether this fixes it. I’ve had this issue in the past, and ended up doing a recovery install to fully fix it.
It seems to happen randomly. Some of them show that icon in the /Applications folder. Sometimes an app will switch to that icon, then switch back to its normal icon at a later point.
Rebooting doesn’t get it.
Try booting into Safe mode, maybe an icon cache got corrupted.