Coding a car is a misnomer. Unlike flashing your ECU, where you are uploading a new program, which could easily brick your car if you do not know what you are doing, ‘coding’ your car is not programming at all, but just changing the values of set functions.
That being said, coding a car is an exact science and you need to follow the directions to the letter or you risk putting your transportation at risk.
While you cannot ‘break’ your car beyond repair, there are several things bad things that could happen if you do not follow things to the letter, including: