You get encoding artifacts when you compress music with a lossy encoder (MP3, AAC, Vorbis, etc.). An encoding artifact is a way for you to tell the compressed music from the original. There's many different kinds, actually. A difference in how a particular frequency is reproduced (or not reproduced at all), a glitch in the sound, an unwanted echo, and so on and so forth.
A glitch could be due to bugs in the encoder. Unfaithful reproduction of specific frequencies can depend on the quality of the encoder, its specific tunings and so on...