Way back in 1962, when I was a high school oboist, my band director gathered up a few of us and took us to the auditions for the Southern California Honor Band. I had practiced the excerpts and prepared a movement from a Vivaldi oboe concerto, and was pretty sure I was ready. So, it finally got be be my turn to play for the panel. I introduced myself and said what band I was from, and a friendly looking man asked me to play a two octave major scale, slurred up and alternating two and two down. "What key?" I asked. He said, "Whatever you prefer." So, I very nicely played my two octave scale in C.
What happened next was quite shocking: "Now play a two octave scale, just as before, but a half-step down." Whoops. Db major is a horror on the oboe, especially on a student instrument that lacks the nifty automatic features designed just for that horrid scale. Fortunately, I had recently added the scale to my practice list, but had hardly perfected it. I gulped, but managed to do it at about half the speed of the C major scale, but without any serious issues, although the slurred version is impossible without the automatic alternate keys.
I played the excerpts they asked for and about half of the Vivaldi and heard, "That's enough. Thank you."
As it turned out, I did manage to get the third oboe seat that year, which also came with the loan of an English horn, which I had never played, but was expected to play in the concerts. Woo-hoo! I loved it.
That was an oboe audition, but the lesson I took from it applies to any instrument, I'm sure. If you are asked to choose your own key for a scale, choose the most difficult one you have down pat. Do not make the mistake of playing the easiest and most familiar scale. Your cruel audition panelist may ask you to do what mine did and give you the most difficult scale possible to play.