While it’s not true that there are no happy accidents in digital art–I’ve had a few myself–certainly they’re less common. Other than an accidental click of the paint bucket, it’s difficult, for example, to simulate the experience of having the glass tube in your eyedropper of raw sienna ink shoot out of the rubber squeeze top, showering raw sienna in a flood across your painting, your leg, and the floor. The annoyance of having to get acrylic ink out of the carpet before it sets, however, is much mitigated by discovering that the effect when you wipe the ink off the painting is more or less what you wanted anyway.
There is also the experience of deciding that you want to play with the impasto brush in Painter, having the program crash to desktop ten minutes on, and saying “FINE! I’ll go do it with REAL paint, you sonuvabitch, and lay it on with a trowel!” but I’m not sure if that’s a happy accident or just a further manifestation of my snarly muse.