The text plays on the double meaning of the word 'bug'. In computing, a 'bug' refers to an error or flaw in software. The joke redefines a 'bug' as an 'undocumented feature', which humorously suggests that the error is actually an intentional part of the program that just hasn't been documented. This creates a humorous reinterpretation of a commonly negative term.
It ' s not a BUG , it ' s an undocumented feature !