I've been gaming with the same core group for about 12 years or so now (we've had some changes to the group, but there have been two or three constants the whole time, and everyone who plays in our group is very familiar, if not very friendly, with each other). We've had a few cases of bad DMing (myself included, unfortunately):
- One DM had a very cool scenario developed where he wanted us to fight a dracolich. Our group was pretty terrified of this, so our wizard used a "control undead" scroll which, by the rules, would have let him control the dracolich and send it away. The DM didn't want that to happen and just ruled that the scroll didn't work and we had to fight it anyway. It did make us pretty mad at the time, but he's still one of our best friends (and a very, very good player), and he's since apologized for it, which is cool with me.
- We had a DM who had his own world. It was a very detailed world that he has spent years tinkering with in various forms. One of the biggest problems was the dreaded DMPC, which followed us around for literally the entire campaign, and it was hard for our PCs to really feel cool around him. However, I think he started to realize it was a bad idea, and the character ended up dying close to the end of the campaign, and it is still one of the most fun, fulfilling campaigns our group had played, and we still talk about certain scenes from it to this day, many years later.
- I'm very bad at roleplaying as a DM. I know this weakness of mine, and my game, which has been running over two years now, has been tailored to suit my strengths as a DM. There have been many sessions where I've tried to do more RP for my players, and it always ends with them half-asleep from boredom. Bad DMing, not recognizing what works and what doesn't.
Basically, we've had our problems, but we've been pretty good at diagnosing them and treating them, and we have a very good group. Fortunately, we've never really broken up over any of these issues.