hi guys,
just noticed that when my arduino 2560 has power 5v external and usb plugged in the microchip is running very hot, causing a small burning smell.
what can i do to stop this, when i unplug the usb it goes back to normal. but i need it to connected to usb , worried this might burn up??
pls help 
one other thing i have noticed is when the usb is plugged in the status led on the board also lights up quite bright
one other thing to mention is i unplugged the board halfway though being programmed, the board still functions as normal. also seems only to be when powered to usb, even on its own the chip heats up. stings the back of my finger its hot. desperate to solve this problem, can i stop the board from being powered by usb???
fixed just by cutting the power line inside the usb cable, usb functions normally, yay 
You are lucky! Some users trashed their Megas powering by +5v and usb.
http://arduino.cc/forum/index.php/topic,82046.0.html
If you read all the posts, you will see that powering by +5v and usb is not recommended.