If I accidentally favourite a tweet (which I do regularly in Tweetdeck for Android), then undo the favourite straight away, what does the original tweeter see when they view their interactions?
You can undo a favorite on a tweet, but depending on that user's notification settings, they'll know that you favorited the tweet in the first place. They might not care, then again they might.
At the Twitter website, track down the tweet (or see the tweets that you have favorited from your profile page) and click on "Undo favorite" (it's next to the "Retweet / Undo Retweet" link)
Favouring a tweet is really more for you than it is for the user of which tweet you've favoured, it's more easier if you look at it as bookmarking a web page, when you are favouring a tweet you are saying, I like this tweet and will want to see it again some time, favouring it then puts it in your favourites for later re-use. So to answer your question: un-favouring a tweet will do nothing for the owner of the tweet you favoured really, because they would already have got the notification, but for you the tweet will be deleted from your favourites.