[nehushtan]

Like others have said, there's unethical deception involved in inserting arbitrary errors - especially to make it "look like it’s been generated by a human".

But my feeling is that in addition to paying the OP to "do a job" the company is also paying him/her (him from now on) to "be on call". Yes, they want X results but they also are paying a salary so that they can tap him whenever they need to. This aspect of the job is referred to when he says there "might be amendments to the spec and corresponding though email".

To some companies (especially those with very little other in-house expertise) having "the computer guy" on call to handle all of that mysterious stuff is worth a great deal of money. The company could consider it their insurance against catastrophe.

Nevertheless I would say the OP should come clean at the next performance review.