If you stand on broken glass without shoes (and socks) glass shards get stuck on your feet that need to be removed with tweezers or by hand. Once you remove the glass shards, new ones get applied immediately. You can park your character on top of the glass shards and keep on removing glass from the feet to get first aid xp. The damage taken from standing on the glass is quite minimal so it is possible to have your character standing on the broken glass for long time.