Say, "You did it wrong."
Say, "Hey, how's it going? Do you need any help?"
They'll say it's done.
Thank them for getting it done. Say, "I have a few questions."
Tell them what the problems are in as self-deprecating a manner as possible; for example, instead of saying "You forgot the widget," say, "I couldn't find the widget. Am I looking in the wrong place?" Eventually they'll figure out that they did it wrong. Then, be sure to thank them again.
If I ever slip up and tell you that you did it wrong, then please don't be offended. Rather, take it as a sign of my respect for you -- I'm trying not to waste your time.