Anyone who creates things for a living knows that it's hard to feel great about everything you make. Even if there's nothing wrong with the content you've produced from an outsider's perspective, creating something from scratch and being with it every single step of the way makes it easy to spot those minor flaws, those small details that you wish you had the time (or the foresight) to change.
So, imagine working on something like Doctor Who, a show that has millions of fans counting on you to deliver something great, week after week, year after year. It's impossible to keep a consistently high standard on a complex show like this, and nobody knows this better than the creators themselves.
Even though it's quite rare, there are times when the show's various actors, writers and producers have opened up about certain moments - usually moments they were responsible for creating - that they just don't like, whether it's a single line, a single scene, or even an entire run of episodes.
Nobody can strike gold 100% of the time, and whether you agree with them or not, Doctor Who's production talent sometimes has negative things to say about the show they help create.