Going into the underground to try and get the necessary stones needed to evolve a couple pokes and to get the fossils. Could be down in the underground all day and not get 1 moon stone or a helix fossil!

I'm trying to chain for Ponyta shiny, but I sux!!!
Even if I'm doing it right, a random pokemon appear and break my chain...
My max for now is 16 in a row, 2 times.

my most frustrating thing in sinnoh were :
1)the constantly appearing
2)crossing the reverse dimention
3) lack of s s s
.
.
but the game was cool

The only frustration that I can distinctly remember is trying to train my Monferno during my Pearl story play-through. I remember how ludicrously long it took me to grind out levels, due to that offensively abysmal, pre-Infernape learnset. It could seldom kill anything on its own, so it had a big drag on its Exp. earnings for the longest time. Was worth it after it finally evolved, but I haven't before or since had that much difficulty in training a starter.