Four laboratory studies show that people are more likely to overlook others' unethical behavior when ethical degradation occurs slowly rather than in one abrupt shift. Participants served in the role of watchdogs charged with catching instances of cheating. The watchdogs in our studies were less likely to criticize the actions of others when their behavior eroded gradually, over time, rather than in one abrupt shift. We refer to this phenomenon as the slippery slope effect. Our studies also demonstrate that at least part of this effect can be attributed to implicit biases that result in a failure to notice ethical erosion when it occurs slowly. Broadly, our studies provide evidence as to when and why people overlook cheating by others and examine the conditions under which the slippery slope effect occurs.