Have a look at the following dictionary entry, line 5:
Who's bright idea was it to leave the refrigerator open?
Source: Merriam-Webster’s Advanced Learner’s English Dictionary under the entry: bright
Is that an error in the dictionary? Shouldn't the example be as it is on the online dictionary:
Whose bright idea was it to leave the refrigerator open?
I'm guessing the example shows how people confuse whose bright idea for who's bright idea, but the dictionary does not mention that.