Autonomous Line Following Hovercraft

Hi guys
My team is working on a line following hovercraft. We are left with the line following part. For that we decided to use an Arduino Uno board.
The problem is that nobody in our team has any experience in working with arduino. As a result 4 arduino boards got damaged.
We are using a brushless motor for providing lift, servo motor to control the rotation of rudder, 3 IR sensors to detect the path. The signals from the sensors are fed into A0,A1,A2 analog pins of arduino board. The pwm pin6 is connected to the servo motor. As we are using a brushless motor we need to use an ESC. A 30 amp ESC is being used which is powered by a 11.1V 1300mAh Lipo Battery. The signal pin of esc is connected to pin5(pwm) of arduino. The ESC and the 3 sensors are powered by the 5V Pin of the arduino board. Also IC 7806 is used to deliver 6V to servo.
Initially the arduino board was working fine(power to the board was supplied from laptop) as we were testing the rudder,sensors,motor separately and we were able to upload the code as and when required. At one point of time we thought of providing a common ground for all the components.
So we disconnected the USB cable from the arduino board, took a wire and connected the battery gnd to arduino gnd pin.
(remaining connections were intact). To our surprise the arduino board started functioning without any input power being fed.
(Vin pin of arduino is not connected to battery). And post this incident the board got damaged. We weren't able to upload the code as the laptop was unable to detect the serial port."SERIAL PORT COM6 NOT FOUND" was the message appearing every time we tried to upload.
Then using a multimeter I measured the drop betweenVin and gnd pin of arduino(connected to battery gnd)(connections intact as mentioned before). It showed 8V(without Vin pin connected to any source). And I found out that ESC was supplying power to the board.
I am unable to figure out how can ESC power the arduino and what was the reason behind board's damage.
Can anybody help me out?????

Waiting for replies............

well, very impressed,
hovercraft dont have breaks, so are VERY difficult to control,
you must be doing some impressive Kalman filtering ..

so, blown uno, not to surprised,
try these guys,

a bit more what you want.

probable reason the esc is driving the arduino is the esc has a pull up on its input to the vsupply.
so your driving volts to the arduino, the CPU on the arduino has built in diodes to its power and ground, from the io pins. so your volts get put to the arduino, but as its 8v, its goign to blow the arduino,

have a look here, method 3...

and no I'm not connected to these guys, we just use their boards, and despite my students best attempts, they all still work.

As you've discovered, you shouldn't supply power to sensors and other devices, while connected to an unpowered Arduino (or any microcontroller, for that matter). In that case the sensors can provide power through the input/output pins of the microcontroller, because there is an effective diode on each input/output, connected to the microcontroller Vcc line. If a significant amount of current passes through that diode you will damage the chip.

In effect, you've violated one of the "absolute maximum" rules on the microcontroller data sheet. Look for a table of maximum input voltages etc., where you will find an entry something like "Vin not to exceed Vcc+0.5 V". But Vcc=0 in your case, so this is not hard to do.