Get Everything In Writing

Psychologists at Washington University report that   the memory function of people in their mid-60s and up is easily swayed by the   power of suggestion, making them more vulnerable to memory-related scams. A   full report appears in the May issue of the Journal of Experimental Psychology   (JEP): General, which is published by the American Psychological Association.

For example, an unscrupulous contractor can tell an older customer, “I   told you it would cost [a much higher price than was originally quoted] and   you agreed to pay!” Without a written estimate, the customer is likely   to “remember” it that way, too, and be overcharged. Another scam,   “you forgot to pay me,” also underscores the need for written receipts.   Some con artists even run their own rudimentary memory tests on the elderly,   checking to see if their memory is bad enough to make them good victims.

Get the full story here.

My own experience (of being on the receiving rather than the perpetrating end) suggests that you don't have to be that old to not always remember clearly what was said - just being preoccupied with something else and not giving something your full attention will do it.