The only time I've had a problem is when I've panicked because of a road issue such as a pothole, or bad iron-works; you immediately want to freewheel so you can sort yourself out, and if you can't, it can get messy.
My best ever moment on a bike was on Fixed, on one of the Ut's rides at Henham, when I encountered a ride-ending pothole whilst doing 50km/hr downhill. I sent the bike to the left of the pothole, whilst my bodyweight went to the right - all whilst still pedalling at 120rpm. No idea how I did that.