So, one of my latest pet peeves in fanfiction is sudden and unexpected crossover.
I remember several SI/OC in Naruto, HP, and Worm fandom in which MC is this brilliant, skillful, and smooth operator due to being inserted into an actual brilliant, skillful, and smooth operator character's head.
Like Kakashi or Tom Riddle.
This character then proceed to monologue about how great he is, and take action to better his/her position.
I'm still fine with it at this point.
But then suddenly MC either crack dimensional travel to other fandom, off screen, or just managed to create power/skill/magic from other fandom. Again, off screen.
This is then used to somehow be the justification for opening the faucet of the sudden crossover.
My pet peeves is, in short. Why?
If you write an SI in a specific fandom, usually you already know, or at least, has a vague direction, on what you, as the MC, want to do in the setting.
If you perchance decides that discovering dimensional travel or summoning outer god or recreating other setting skills is what you wanna do, than at least shows the effort your MC does to achieve that.
Not just suddenly giving Kakashi from Naruto the UBW from Fate, or making Tom Riddle from HP becoming Le Fay from Fate's mentor.
The change is just jarring is all.
++++
My second Pet Peeve of the day is when MC bring religion and modern sensibility as basis for hypocrisy in "historical" setting.
Especially the one in which MC is born as part of conqueror/raider tribe and be forced to "partake" the loser tribes woman and then spend way too many words angsting about it.
If it offend your sensibilities, take over, and change the culture.
I especially hated when religion is brought upon.
There's a part of me that dislike stories about religion, due to my upbringing in which most people is somewhat a devout.
However, that part is so small compared to the fact that interesting stories that brought up religion tend to invite backlash, that in return results in either flame war, or a ban.
Either way, the stories are dead anyway.