Something curious: In Argentina it is common to say in an informal conversation "[Esto] es un horno" when you are in a very hot room with no fresh air or when you go into your car which has been parked under the sun for hours
When it is the difference between "a" and "an" it's darn near impossible to tell if the you made a typo or if you used an incorrect article... so it's better to err on the side of caution and just mark us incorrect in these instances.
The hint when I click on "horno" says he Translation is "oven"
However the correct answer is "furnace", and"oven" is marked as a mistake...
I know the difference between an oven and a furnace, but the tooltip is really irritating