Think of it this way: You fix a flat tire of a persons car and as soon as they drive out the engine explodes! Would the person fixing the tire be liable? Not in this case if it went to trial.
OK, now lets use your example remember you are doing brain surgery here ;-} Sadly, you are taking the risk if something else breaks in the process. Again, think of what would happen in a court room you would be found liable? Liability isn't just what you repaired it can be the other damage you created if the repair was proven to be the cause of it lets say a fire or the death of someone. This is not to scare you but to make sure you have the needed protection and take care when repairing things.
As an example some iPhone chargers in China where shocking people (they were very good knock offs of Apples design) Apple was forced to deal with quite a few lawsuits and not only had to offer free replacements of the chargers to make sure they wouldn't have to fight still more lawsuits! Remember they didn't even make the charger or sell it! Before this they had to deal with exploding iPhone batteries from a few knockoffs that clearly weren't up to snuff.
Just because you're small doesn't mean your safe from someone suing you.
Sometimes its just best to service the customer even if you don't think your the cause. Today people have social media to either praise you or vilify you. Consider that as well, a bad rap will hurt you in the pocketbook.
Make sure you have a good trail of documentation (even pictures) of what you did and if you do have someone make a claim of poor workmanship make sure your workmanship is above reproach. If not, you need to rethink if you have the correct skills & tools or you need to improve them.