My favorite is a question that asks you to check the box next to all the conditions or situations that apply. The survey provides, say, 5 or 6 different responses, none of which applies to your particular situation. So you leave the question blank then go onto the next question, finish filling out the survey, then click the SUBMIT button. For this, your reward is, "You didn't answer question 8. A response is required before you can submit your survey." Great, you at least checked my answers but, because of poor survey design, I can't answer one or more of the questions properly.

For those of you designing Q&As or survey, take care about the questions you ask and the responses you allow. That you caught an omitted answer is fine; that you didn't allow an answer you didn't want is directing and poor survey technique.