A similar phrase that first springs to mind is loose cannon, which etymologically has little to do with a cannon's firepower, but more to do with the dangers of a unsecured cannon rolling around on the deck of a ship. Idioms using the word glass can refer to fragility (as in glass cannon), but the word often connotes transparency as well. More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.