I have had happy accidents in which I've moved clips around haphazardly on the timeline, sometimes just to get them out of the way temporarily, and then later listened back and found that a bunch of parts that I never intended to play together actually compliment each other very well. Or that parts I'd lined up to play together actually produce a far more interesting harmony when they've been accidentally offset by a few beats. Another thing I'll sometimes do is move a MIDI clip from one synth track to another accidentally (by intending to move it horizontally without realizing it's also moved vertically) and will find that the MIDI works even better on the new synth.
Great synth sounds often come about by happy accidents too. I very rarely start with a set idea of how a synth is going to sound. One thing I really love is synths which interpret incoming MIDI in a peculiar way and pump out something quite different and unexpected. Lots of happy accidents like this. Native Instruments' Rounds is one of my favorites for this. Sometimes when I get disillusioned with a MIDI part that's being played with a more conventional synth sound, I'll direct it to Rounds and start messing about with it. It gives you some very interesting results.