Both of them snapped cracks in the panel just as the drill bit got to the other side. I don't know if I screwed it up or if it's because the glass was pretty thin. It occurs to me I've never drilled anything thinner than 4.5mm before (this was 1/8" or about 3mm thick I think).
I feel pretty bad about it.
I've drilled easily 20-30 holes before and only had glass break on me in 2 or 3 cases, and usually there was a reason. One time it was because I lost my balance and fell forward onto my sump as I was drilling it. Another time was because I was drilling on the vertical into a running tank, probably the riskiest scenario of all.