Believe it or not, it is possible to make an LED emit smoke (No, I am not kidding!). During my electronics class in high school, we had a lot of spare time and minimal supervision... so we would get LED's out of the spares drawer and connect them to an 18V power supply with no regard to polarity or current limiting. Most LED's would just pop and burn out immediately, with a faint whiff of melting plastic. Some would shatter, spraying hot plastic everywhere. Out of about 60 LED's that we blew up, two actually ignited and caught fire for a brief period.