Generally speaking, the employer wins more. This sounds bad for the contractors, but unfortunately the contractor has a greater burden of proof. If a project goes in for 100% testing, the employer has only to prove that the bug exists and the fixing it was part of the contract. The contractor has much more to prove – they have to show that the project works without error.
This isn’t to say that contractors never win, this is far from the case. Employers have to give the steps to re-create the error and show that they had informed the worker about the problem.