@ deadastronaut:
it happened to me when I did my first "reversed" etch. I followed my usual etch routine, but there was much more aluminum exposed, especially because at some point, the etchant overflowed and went into the box instead of being only on the outside. The FeCl was straight from the bottle, so it got really hot really quick, it boiled and relase a really dreadful smoke.
I panicked and flushed the acid bucket with the water bucket to stop it.
Very scary and probably not good for my health
Somehow I think when a lot of aluminum is exposed the solution and the box get hot really fast so it gets into a "spiral".
Now, I dilute the solution when I do a reversed etch and wear gloves, a mask and goggles, even for something small. I don't want that to happen ever again.