Dude just search "iron left on starts fire" or similar on google. flicker even has pics of the metal melted on irons that went up in flames.
http://www.london-fire.gov.uk/LatestIncidentsContainer_26jul12AV.asp
heres a lil something to read.
even here it says that the fire stareted was because clothes near by got lit, I read one more story, it said that the plastic itself melted on the iron thus causing a fire this time the electricity continued flowing cause the circute didn't fail if it would have been a better quality plastic it wouldn't have melted, also there are safety mechanisms that switch off when it gets too hot, most people who had left theyr irons straight up not touching anything, find everything the same, or as i said the electricity gone in a part or the house, the fires are cause either because someone was in a hurry and left it next to something flamable, or by cheapass irons that melt themselves .