I think that by the time that anyone is thinking to call a lawyer, the job has already failed.
Since this thread is about contracts, I'll focus on that element.
Here's where an ounce of prevention is worth a pound of cure. Make sure from the outset that you're clear on what you need to do - and that the customer is clear on what to expect. That's your primary way to avoid problems.
Avoid the temptation to 'pass ther buck.' Yes, I know this means you're biting off more than some guys want to. For example ... if you state that wall repairs are the customers' responsibility, and they're not happy with the repair, they'll associate that with you. That will get them looking for other faults. Pointing to the fine print of the service agreement after you've made the hole is no way to have a happy customer. Make sure they understand these things before you make that hole.
Recognize that things get complicated when intermediaries are involved. For example, when you're brought in by the general contractor and the customer is present. Or, when the owner hired you, but his manager is present. These 'intermediaries' weren't there when the deal was made ... but they sure can muck things up.
