Sometimes (and seemingly randomly) application icons don't show the application-specific icon for the application (see image). This happens everywhere, as far as I know (dock, application switcher, viewing
/Applications in Finder).
I think this may have started after upgrading to High Sierra. I thought it would go away after restarting, but it hasn't.
I also recently added some of the OSX defaults in this script to my dotfiles init script, so it may have something to do with it: https://github.com/mathiasbynens/dotfiles/blob/af852609ca208a8d6114af1ae6b2390d57f7dd98/.macos
Here's mine for comparison: https://github.com/jpalmieri/dotfiles/blob/2d7ab788e491cc67e079f5fc243d05d2c49e2e98/macos/set-defaults.sh
EDIT: After restarting again today (a few days later), the issue seems to be resolved. Still not sure why this happened. If it happens again, I'll update once more.