Label: Lesson learned
When you estimate for maintenance projects, i.e, adding features to an existing project (software, website etc.) make sure you consider a pre-testing time also.
Once you finish your task, any issue on the product, even if you have not touched that specific module, will come to you – if you don’t have a previously installed product exists for which you can show the customer by reproducing the issue.
For standalone products this case won’t come as you can prove it by trying to replicate the scenario on another computer. But for website projects, you may not be able to prove your innocence.
So best approach is to test the product/project and prepare a list of issues and get the formal approval from customer. So in future if you get a bug report, you can verify this list so that your side is safe.