'Random' leaps aren't there haha! Chopin's Op 25 #4 is a nightmare for me.
I used to think double notes bugged me the most, but you see them so often in rep that they are difficult to hide from. Random leaps on the other hand, DAMN. My first encounter was near the end of Rach Op 23 #2, when the right hand has mammoth chords and the left hand is in octaves (before the 32nd note cascading passage). That took me WAY more time to work out than most double note passages... relatively. Obviously if you take a piece like his Prelude in Eb minor (which is 4 pages entirely of random double notes), that would be tougher. But 4 pages of the leaps in the Bb would be hell.
Double notes come nicely if you work on the articulation, and leaps [for me] just take repetition at a very slow pace, sinking in to the keys. I find leaps more frustrating to practice, because it is walking on thin ice when you increase the tempo lol. I find it can collapse so quickly.
But ya, my ultimate nightmare would be a prof saying "ok, this year you will play Chopin's Op 25 #4"... bleeergh.
Double Thirds Glissandi are up there (I played Alborada two years ago), but it's such an obscure technique. What I actually found more difficult was coordinating the left hand rhythmically underneath... that sucked. The descending glissando was awkward if your hands were a bit sweaty lol, and was a PAIN for the outer finger, but holy sh*t getting the rhythm to cut through in the LF is worse in my opinion.
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On a side note, another technique that bugged me when it came up was the variation in the Beethoven 32 Variations where both hands had the triplet broken triad figuration. Ack that was bad, and I had to prepare that piece for a summer course in France... needless to say I embarrassed myself the first time I played the piece lol!