An elderly couple

An elderly couple is getting ready for bed.
She says, "Oh I am just so hungry for ice cream and there isn't any in the house."
He says, "I'll go get some."
She says, "Vanilla with chocolate sauce, with whipped cream on top and a cherry."
She adds, "Please write it down, I know you'll forget."
He says, "I won't forget; Vanilla with chocolate sauce, whipped cream, and a cherry!"
Away he goes.
Hours later he comes back and hands her a paper bag.
"In it is a "HAM SANDWICH".
She says," I told you to write it down! You forgot the mustard."