One of the most common reasons why a project might fail is overconfidence. When we are overconfident, we tend to believe that we are properly prepared to do more than we actually are. We can build expectations that are not backed up by reality.
An example of this would be putting on a play in a theatre. A play requires a lot of preparation, as well as lots of practice. After a few practices, you might get overconfident and believe no more rehearsing is necessary. This could backfire if you are not ready to present the play. A thing you could do differently would be rehearsing much more and being more realistic about your expectations.
