I have on many occasions amended a form 1040 that I did not originally prepare when someone brings me the original return and the document they forgot to provide the original preparer. "Hey, Preppie, I forgot to give my 1099-R to my preparer and now he's dead (or whatever). Can you amend this tax return?"
I look at the return, verify there is no 1099-R already on it, and then produce a 1040X with the 1099-R added in.
Today someone asked me to amend their 2016 1120S. I did not prepare the original. She gave me a copy of the original return and a copy of an invoice for ~$26K of goods purchased stating, "I forgot to give this invoice to my CPA."
It seems like a very bad idea for me to amend this return without seeing ALL the documentation for 2016. Would you amend with just a stray invoice, the original return, and her word it had been left off the original return?