Instead of simply answering this question by recounting a time when I made an assumption, I want to discuss types of assumptions that you might have made that would come back to haunt you. It would not make sense to just tell my own story because that story is clearly not one that you could use for your assignment. Hopefully, I can point you in a direction that will allow you to tell your own story.
One important kind of assumption is an assumption that you make about other people based on their appearance or some other fact that you know about them. Such assumptions can lead us astray because people are not always what they appear to be. For example, when I was going away for my freshman year of college, I found out that my roommate was going to be an African American from Detroit. I was happy because I assumed he would love basketball like I did. As it turns out, he assumed from my picture and name that I was Hispanic and would love soccer like he did. We were both wrong.
Another sort of assumption is an assumption that you make based on previous experiences that might not apply to your current situation. In other words, you might assume that something that worked for you in the past will work for you again in the future. You might be missing some important difference in your current situation that will make your assumption invalid. My own example of this comes from my decision to go to graduate school. My grandfather had made a career as a college professor who taught classes and did not do research. I assumed that I could do the same. I failed to find a job after getting my Ph.D. because I had failed to understand that colleges no longer have any interest in professors who do not want to do research.
Perhaps you can think of assumptions similar to these in your own life. Thinking of such assumptions will allow you to write a response that is authentic to your experience.
This may get quite personal, but I had a friend when I was younger who stole from me and treated me badly. I didn't want anything to do with them anymore, but then they apologized. I felt alone because they had turned most of my friends against me, so I accepted their apology. I immediately knew that was not the right decision, but I didn't know what to do. They basically bullied me and for a while I did nothing about it. I finally got the courage to stop associating myself with them and it made my life better. This also helped me learn how to tell who my true friends are and I think that's one of the most important things you can learn in life. I hope that helped you in some way :)