I don't really think the title works for this, as canon would refer to a published work, but meh, here we go.
Something I've noticed in roleplay is that when people randomly jump into an rp you are doing, or you let them join, sometimes their characters will break the "laws" so to speak of the original roleplay. This can make things very annoying.
The other day I was roleplaying with a friend in a private map (that is public download). We where continuing along a roleplay we had been doing for a while, basically an Area 51 type thing with our alien characters. These characters and the roleplay in general had a pretty strict sci-fi theme (in other words we tried to avoid breaking too many laws of physics xD).
So then another character shows up and sees us roleplaying. She asked if she could join, and not wanting to be rude, we let her.
Turns out her character is a several million year old immortal magical two-tailed canine that could withstand being thrown in a tank full of lava.
It completely ruined the theme of our roleplay. Later a few more people came along and joined in the roleplay without asking. Many of them had "magical" powers and such.
We didn't want to be rude and tell them they couldn't rp with us (we considered moving it into party a few times, but eventually decided not too), but it still broke the laws of the "universe" that governed our rp.
Just want to hear your thoughts on this kind of situation.