The General Rule
Mechanics are required by regulation to follow the instructions set forth in the manufacturer's maintenance manuals when working on an aircraft. The mechanic is not allowed to deviate from the instructions covering the work he undertakes. If he does deviate, and someone is injured as a result, the mechanic is liable.
Sometimes, a manufacturer learns of a problem with the way its product is performing in the field.
The manufacturer may then issue a “service bulletin” to warn the industry of the problem and how to correct it. For example, when it learned that its exhaust valves were failing at an unacceptably high rate, one engine manufacturer issued a service bulletin requiring that those valves be inspected regularly (pdf) and, if necessary, repaired or replaced. The service bulletin warns that failure to perform the inspection can result in engine failure. Because the risk is so great, the manufacturer labeled this particular service bulletin "mandatory."
If a mechanic works on an aircraft but returns it to service without performing the "mandatory" inspection, is the mechanic responsible to those injured if the engine quits shortly thereafter due to valve failure?
You might think so. After all, of all the manufacturer's instructions, those in the service bulletins might be the most important. However, the regulations allow mechanics to ignore service bulletins entirely, even “mandatory” ones, and still pronounce the aircraft to be "airworthy." That is, at least when working on aircraft operated under Part 91 of the FAA regulations—the section under which most general aviation aircraft are operated.
The Industry Practice
When an aircraft is brought into the shop for service, many mechanics provide the owner with a written list of the service bulletins that apply to the owner's aircraft. Unless the maintenance manuals expressly require the mechanic to comply with all service bulletins, the mechanics leave that decision to the owner. After explaining to the owner the work entailed, its cost, and its importance to flight safety, these mechanics require the owner to direct them, in writing, either to comply with the various service bulletins or to ignore them.
The mechanics keep the owner's written instructions on file. In the event of an accident, the mechanic will argue that, although he was the one who pronounced the aircraft "airworthy," it was the owner who decided not to follow the manufacturer's recommendations. The mechanic will argue that therefore the owner, not the mechanic, is the one responsible for the resulting injuries or death.