This is the first time I've seen this, but @Catrope has seen something similar, but with different icons.
I could understand if the font didn't load, but aren't these supposed to be in a private use area? I would think we would get rectangles in that case. Maybe there is another font conflicting, with a load order issue (and usually it's the lucky order, but not always).
This was in Firefox 31.6.0 on https://www.mediawiki.org/wiki/Talk:Sandbox .
(Granted, they do have a cool non-representational art thing going on).