While doing the random missions I got one to plant a gem in my wife's house in whiterun... I've never had this happen before and I have over 800 hours in skyrim.

I never bothered to check but what happens when you plant evidence in someone's house? I don't think I noticed anything but like I said never bothered to check. I'm going to deny it anyway for rp reasons but it got me thinking about what happens.

On a side note what happens when you quit a side quest from them?

Nothing, those quests are randomised I think so if you quit it and ask for another one they should assign you a quest to a different NPC