@klavieronin: Immortal British humor!
This reminded me somehow of this:
But to give furiuzpianist a serious answer:
the opening trills of K330. If you mess up this brilliant opening, somehow you spoil the charm of the whole movement.
also hard, is calculating (yes it needs calculation) the levare of the "easy" op.25 No.1, so that it sounds like a levare but entering into a melody played piano, with even quieter arpeggiations underneath. It's tough to get the right touch from the very beginning even more so if you haven't warmed up and/or playing on a piano you don't know.