Porn example: Sara Stone on Naughty America. After being gone a few years, she returned to the biz with a "exclusive" deal with the site. As expected, she had a couple new scenes... then more and more and more... to the extent that if there was a possible sub-site/genre she could be placed in ex. (threesomes, cowgirl as in the occupation, cheating wife, etc.) you could best believe she was there, sometimes two scenes of her per week. This isn't to say that she was actually bad in said scenes, but the viewers called the site on it and eventually the frequency died down along with the exclusive deal.
Certain Neopets rank low in popularity, but seem to be favorites among the creators. The chia (currently ranked 31/54) and blumaroo (36/54) both pop up far too frequently on the site in the form of NPCs, special items, location art, and the like though at the very least the chia was the first neopet ever created.
The biggest example on the site is probably AAA, the game master. Not only is he an insufferable prick, but his challenges also directly cut into the various plots on the site, forcing them to be rushed. Considering that the plots and their associated puzzles were some of the highest draws on the site, many fans cried foul.
The Furtopia Darwin's Soldiers RPs has Dragore, who fits this trope perfectly. He is a major Mary Sue and a scrappy with a big role, adored by the creator and steals the spotlight.
Lupis from the rebooted Furtopia RP is even worse. He literally hijacks the entire story and the plot shifts from a focus on the terrorists to the rogue scientists, who were originally in the background.
Ronnie the Skeleton on The Irate Gamer. He is a Jewish stereotype who was hated from his first appearance, where he was given an extremely Overly-Long Gag regarding talking non-stop on the telephone. It seems none of his fans or non-fans like him, yet Bores is attached to him so much that he'll put him in any possible place.
One of the recurring complaints about Marvel Super Heroes: What The—?! concerns the fact the ugly and obnoxious MODOK appears in nearly every episode. Despite this, Marvel still gives him a frequent amount of scenes.
The Alchemy feature on Gaia Online. Since it's release in 2011, the feature has been completely reviled by the userbase. Alchemy components are nearly impossible to get, and the only ways to obtain them are luck-based. For recipes for items that existed on the site prior to the release of the feature, getting all the components and paying to craft the item costs far more than just buying it on the site's marketplace. Even the richest users on the site have to work together to make some of the items. Nevertheless, much of the Gaia staff insists that the feature is SUPPOSED to be hard and that it's working as they intended.
