This is caused by your game client not loading specific image assets (in this case, for quoins), but the reason it didn't load those assets is less easy to ascertain.
Did you try reloading your game client to see if that made them visible again? Also, if you're at the same location you were playing from when this happened, can you load up alpha.glitch.com/help/test/ and tell me if you get any errors on that page? There are a few possible causes, but it'd be good to narrow down connection to the CDN as a non-factor.
I'll try to remember about that test page. (Seems like you all might put a link on the home page or something? I don't remember hearing about it before.)
We've seen a few similar reports of this so we're looking into a number of possible causes. It's a tricky issue to debug because it appears to reach through 3 or 4 different layers of software (only one of which we have direct control over), but we'll get this figured out and fixed.
That test page will be more visible soon; we just completed it recently so it's still being integrated into the rest of our help/support/QA systems.