SQL NULL. Javascript NaN. Their behaviour is defined by the standards and special checks are required to identify these values properly. In SQL, it's X IS (

SQL NULL. Javascript NaN. Their behaviour is defined by the standards and special checks are required to identify these values properly. In SQL, it's X IS (NOT) NULL , in Javascript it'd be isNaN(X) . rand() (well, most of the time[1]). This you just have to know :) Improperly expanding C macros: #define isupper(c) ((c) >= 'A' && (c) <= 'Z') #define X isupper(getchar()) printf("X == X = %d\n", X == X); And like lurker14 said, anything in general that has side-effects. [1] It is possible rand() will return the same value twice in a row, just not very likely. Also the original author might have been twisted enough to memoize it. Thank you, "Higher order Perl", for that idea.