Mathematics texts sometimes ask questions that are missed by careful editors, with the result being an unintentional open-ended question.
A Riverside Math textbook included a figure involving two lines (AB and XY) crossed by a transversal (CD). The question asked: "Name a pair of interesting lines?"
Now, that is an "interesting" question...and very open-ended. My answer is the pair WS and QR.
Similarily, an Addison-Wesley text included this word problem: "Patrick worked in the laboratory 8 1/2 hours one week and 10 2/3 hours the next week. How many more hours did he work the first week than the second week?"
Now, is the answer 0 hours more? Or is it - 2 1/6 hours more?
If you know of other "errors" such as these, please send them to me. For some reason, they make my day.
Source: Scholastic Math, 3/17/1989 and 2/16/1990.