You've read all the books, you know everything there is to know about blacksmithery in C17th Wiltshire. And now you're going to share it with us. Sometimes lots of information is good: Frederick Forsyth in Day of the Jackal writing about how to get a fake passport or smuggle a gun through customs. But most of the time it's bad.
The main character's funny cousin, the doleful postman who delivers the fateful letter, the whacky best friend. All tertiary characters ie they appear but don't do much plot-wise, should be kept on a tight lead and not allowed to take over, however hilarious you find them.
Digressions and hobby horses:
Following ideas as you write can be a very creative process, but it can also lead you way off the plot. Similarly, you may have strong opinions on many subjects, but a work of fiction is not the place to sudden start spouting about the iniquities of the planning system or the unfairness of post code lotteries in the NHS. I once hung on doggedly to a little bit of social satire until I had a brief note from my editor: "What is the relevance of this to the story? Please cut it now."
The scene that's there just for the joke at the end:
Some years ago there was a joke doing the rounds of the the internet. It was about a woman going to her gynaecologist and realising that she'd last washed using a facecloth that her child had used to store some glitter. Not too long after being sent this joke I read The Adultery Club by Tess Stimson, where the main character goes to see her gynaecologist and realises... There's no real reason for the scene to exist except for that joke. Sometimes it's a quirky name that's given just for the purpose of people making a joke out of it. I once called a character John simply so I could make a Dear John letter joke. It never worked, and got culled in the final drafts.
The simple test for spotting these darlings is to ask yourself: does it serve the story? If not, then murder is the only answer.