These are signs of some of the problems that can result:
- Abuse of Armed with Canon.
- Continuity Porn for even the most minor things. Writers will revisit old stories, instilling far more self-indulgent detail into the retellings than ever appeared in the original.
- Continuity Snarl when it wasn't part of the story before, and further entanglement of existing snarls.
- Darker and Edgier (and all its variants) on shows, even when it either doesn't fit or is already dark.
- Die for Our Ship incorporated into canon.
- Dropping bridges on characters they don't like, and sometimes making up stuff about the characters to excuse why they did it.
- Fan Myopia dictates most of their choices.
- Flanderization happens to random canonical details that grow out of the original setting's proportions—whether it's compatible with the rest of the old canon or not.
- Lighter and Softer
- Sneaking fanon into official sources.
- Fanservice increases exponentially to the point of the audience saying "No more".
- Never Live It Down shoved on characters that were not canonically treated this way before.
- When they aren't Pandering to the Base, they're pandering to themselves.
- Popularity Power, as well as Power Creep, Power Seep.
- Loads and loads of retcons.
- Writers treat the characters they don't like as The Scrappy, even if those characters aren't thought that way by the fanbase.
- Writers make the characters they like into the Spotlight-Stealing Squad, even if those characters aren't as liked with the fanbase.
- Writer on Board to a degree far greater than happened to the show before.
Note that a lot of these tend to show up in fanfiction before this trope makes them canon, hence the axiomnote at the top of the page.
On the plus side, Running the Asylum can be a cure for stagnation of a series, with the fans bringing in a fresh perspective.