Depending upon your encoding tool, you may have access to a checkbox or number box that controls something called IDR frames.
What are these creatures and what is their significance?
More imporantantly, what's the optimal setting?
Well, let's just say that if you're seeing anything like the random blockiness in the picture below when you drag the playhead back and forth in the video window, you're probably using the wrong value.
Via Olivier NOEL, Nicolas Weil, Robin Good