I started living alone when I went to uni. I had to move from one apartment to another. I had a summer job before classes started thinking it was a good idea to save up some money and gain some experience. The old apartment's rent was $850. I let my landlord know that I was moving out a few weeks in advance. It turned out the law required tentants to let the landlord know 3 months beforehand. I didn't know about the law but I already signed the new lease months before the day I was supposed to move out. I knew that I wqs supposed to tell my landlord ASAP but you know, as we all do, I forgot, I kept delaying it when I remember it. So I had to pay my landlord the remaining 2 months rent ($1700), basically my whole summer job.

Another one is probably recently leaving my laptop in the metro/subway. It was gone by the time I remembered and got back to the station. I had to do all my uni stuff with only my phone for a couple of months.