Audible mp3 artifacts?
Pre-echo on very short and percussive sounds - for example crisp solo drums, or better, electronic drums. The TR-808 rimshot and clave are very short and impulsive sounds, about 5ms long (for the rimshot), and at less than 160kbps their attacks get "swallowed". At less than 96kbps some notes appear to be duplicated or out of place rhythmically because of pre-echo. Worse, each note in a repeated pattern will sound different depending on how it falls within the analysis window. Here is an example.

Try solo vocals too. I think casual listeners are better at detecting anomalies in voices (a kind of auditory "uncanny valley" effect). A classical example used in the audio coding community is the intro from Suzanne Vega's "Tom's Diner".