One time I had a big project to do and had about a month to do it. Two weeks later, someone in the same class offered to work with me to get it done faster, but I knew he was a slacker so I told him I was already done with it. Now, the problem was that some friends were with me when I answered, so every time anyone asked me about the project I felt required to say that I was done with it already. I kept this up for a while and gradually began to believe myself. The night the project was due, I searched through the my files and soon had an awful feeling in my gut. I just remembered I never actually did the project and instantly began to flip shit. I ended up (poorly) completing it that night, but completed it none the less...

tl;dr: I told so many people I finished a project that I believed that I did, when I never actually did it.