Riddles that turn on the concept of knowledge, or a related concept like justification or rational belief. One way around epistemic paradoxes is to regard knowledge itself as an incoherent concept (the position of eliminativism or "intellectual suicide").
The teacher says there will be a test next week, but the day will be a surprise, in the sense that students will not be able to determine the day in advance. A student argues that this is impossible: the test can't be on Friday, because if Thursday's class passes without a test, Friday can be predicted as the day. But with Friday eliminated, the same logic applies to Thursday, and so on, eliminating every day of the week. The student is surprised when the test does indeed happen on one of the days.