These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
That One Boss: In Empire Mode from DJ Hero 2, David Guetta's DJ Battle Mix (MSTRKRFT's Bounce Remix). It only has 6 checkpoints and both players have very little advantage, and no matter how you try, David's trigger happy wits will triumph over you in the end (Granted since his lightning fast wits will lock you out thus giving you a slim chance of winning). The only hope you have is to beat him to the punch and be flawless, or lose. (FYI: Draws don't count)
That One Song: Let's put it this way: If any of the mixes you play are made by the Scratch Perverts, chances are you're not gonna clear said mixes with 5 stars in one go, barring the Easy and Medium difficulties. Case in point, Beats and Pieces, The ''Groundhog'' Beat Juggle, and, god forbid, the ''Galvanize'' Remix. For the latter you'll get a trophy/achievement if you did not manage to 5-Star it on Expert and one if you do manage to pull it off.