Only flaw in the series for me
The only flaw (for me) in this series (my all-time favourite, mind you) is Bill Haydon's death scene -- the last couple of seconds in his life. He falls to the ground, shakes a bit and ends up in a contorted, childish pose. It always makes me laugh when I re-watch the series -- specially when considering how great an actor Ian Richardson was!
I still watch the series about once a year, and every time I enjoy it more than the last time.