Today I had a really bad bad time... I was working with my arduino and suddenly a smoke came out of it and I had to turn off. I have no idea wha thappened but I was using a 5V supply to feed arduino at the 5V pin. I was also connected with USB to arduino and my computer. Could this burn arduino? I mean, supplying energy to arduino using the 5V pin at the same time USB is connected could burn it?
It shouldn't. Did you accidentally short out anything? Can you tell what components have failed.
Hi,
Is it a genuine arduino or a clone?
Can you post a picture please?
Tom.... 
That is one of the standard things that you should not do to your arduino (Powering with 5v supply while plugged into USB. Some people report that it has damaged their board, as happened to you. The doc page does warn against that.
What I recommend is butchering a usb cable and connecting the power supply to that, andplugging it into USB to power it (that way you can't accidentally connect the usb to it at the same time
Lesson -: never wire up anything with the power on.
Experience varies directly with equipment ruined 
No.
I have ruined very little in my life.
Plus it is only experience if he knows what he did wrong, which he doesn't appeared to
Thank you for teaching me this lesson. I saw many people on Google that says what I did was fine... So I ask you a different thing: if I was suplying 12v to my arduino at Vin pin, and connect the USB.Could it also burn my arduino?
No because there is a switch on the Arduino board that would automatically change the source of the power.
Hi,
Can you post a picture of you controller showing us the damage please?
Have you tried to upload a program?
Have you a DMM to see if there is 5V on the 5V pins when USB plugged in?
Tom..
It does not look damaged from the outside except there is a small point on the chip (black point like a hole inverted).
So I was wondering this (which is my last attempt to try to understand what went wrong): I have an LED stripe and I connected it's + to the + of my 12v power supply and the - to a MOSFET. The mosfet has 3 pins, I connected one of them to the mosfet, the other one pin I connected to the - GND of my 12v power supply. The third pin of the MOSFET I connected to a digital arduino pin where I could switch it on and off.
I realized now that I didnt connect the GND of my 12v power supply to GND of my arduino. Could that damage my arduino when it was being powered using 5v pin and USB together?
This is my last attempt to try to discover what went wrong and prevent this acident from happening in the future.
What chip is "the chip"?
@Grumpy_Mike "the chip" is the black square thing on the middle of my arduino.
batata004:
@Grumpy_Mike "the chip" is the black square thing on the middle of my arduino.
Funny man.
Now if you want help what ***** chip are we talking about? Your choice.
My arduino nano has only one chip so why do you want to know which chip? It's the chip, the ONLY chip. The square one in the middle of the board. I dont know if you are drunk but I can only count ONE CHIP in this image https://www.arduino.cc/en/uploads/Main/ArduinoNanoFront_3_sm.jpg
My arduino nano has only one chip
Ah you have a Nano - well this is the first mention of it here. Sorry my crystal ball is on the blink otherwise I might have known that.
I dont know if you are drunk but I can only count ONE CHIP in this image
Well turn the board over and you will see two chips on the other side.
See:-
Now with such a crap attitude why do you expect anyone to help you?
turn the board over
lol
Mike tactfully dodging answering the "are you drunk" comment ;).