I've fallen victim to it many times. I think I understand a situation completely and discover that I was missing some fundamental pieces of the picture.
A couple years ago, I had a student who seemed like a complete jerk. He regularly came in late and seemed so disengaged. He seemed like someone so resistant to learning. Toward the end of the semester, he emailed me and asked to meet with me. I prepared myself for the typical end-of-semester conversation, where all the excuses come out and the dreaded requests for do-overs.
He actually wasn't even doing that poorly in the class, so I'm not sure why I felt so primed for him to ask me for favors. He showed up and started his story out slowly. He hadn't wanted to come see me that day, but his Mom had encouraged him to have a conversation with each of his professors. It turned out that he wasn't there to make excuses, but to share that his Dad was in a hospital bed, dying of cancer. He thought I should know, but didn't want to get pity from me. That had been the biggest thing that had held him back from making the appointment with me previously.