You blame your own lateness on traffic, but a late friend's on carelessness

Psychologists Edward Jones and Richard Nisbett found that people tend to explain their own behaviour by pointing to the situation, while explaining other people's identical behaviour by pointing to their character. Stuck in traffic, you call it bad luck; see someone else arrive late, and it's easy to assume they just left too late. The asymmetry shrinks for people we know well, but persists strongly for strangers.

— Edward E. Jones and Richard E. Nisbett, The Actor and the Observer: Divergent Perceptions of the Causes of Behavior — General Learning Press, 1971