Chemist here, you're right, 50% Hydrogen peroxide is explosive, 70%+ is fine in open air, but degrades rapidly. Hence the brown bottle in medicinal grade peroxide since it is subject to oxidation and light reactions.
As for the title, eh, this is nothing. If you want something fun, use 50% HydroChloric acid and 50% Nitric Acid, mix those together and emits a very toxic fume, dissolve most metals (A lot of fun to drop a nickel or dime to watch it disappear).
I used 35% quite a bit in a former job, it was a calibration fluid for the aforementioned packaging products. You'd sit there and watch the flow measurements changing as it circulated because it started breaking down. Finally worked with the major customers to do a substitute + adjusted output with water.
I've not worked with acids much other than weak stuff like glacial acetic. Nitric did come up every once in a while, but it was used more for passivating stainless flow tubes than as a process fluid.
(post is archived)