Sometimes I come across something that doesn't entirely work right or just doesn't make sense to me and I often refrain from thinking it is a bug in the library or code I am using.
For instance today I discovered that Java's TrayIcon slightly alters some of the RGB values of the image it displays. It is not enough to alter the image but just an R that is one higher or a G that is lower.
I am a self-taught 18 year old (with ~5 years) and so I always think that I am just missing some piece of information of how some part of the computer operates and that is why a given situation does not make sense.
So when you do not necessarily know everything about a given area of a computer and how it operates, how can one identify a bug and know it is a bug with absolute certainty?