Whenever you are allowed to use someone else's Google Chrome, before you leave, go to Settings:Manage Search Engines
ther Search Engines
If they haven't cleared their history, it'll be right there. If not, just type in xnxm or You need to login to view this link
or whatever you desire into the Other category at the bottom.
Much more subtle than changing the home page, or some crappy Facebook status, but the majority of people won't know how to immediately change it back.
Imagine when they are trying to help their little sister with a report on a farm, and they type horses into the url bar and hit enter......
You can also right click in the URL bar