Generally speaking, optimists live healthy lives, achieve well-being and are well adapted in society. Some researchers, however, pointed out that optimism is not always beneficial. For instance, some sort of optimists tend not to assume occurrences of negative events, which may in turn lead to increased failures in everyday life.The purpose of this study was to understand the relationships between optimism and error proneness in everyday life. The results indicated that some aspects of optimism, including pessimism, could predict error proneness. In summary, a person who feels like having bad luck and takes a pessimistic view tended to report cognitive failures. In addition, relationships between failures in using smartphone, as a valuable of objectively measurable everyday life failure, and optimism were also found, but optimism could not predict clacked smartphone screens very well.