Sentiment analysis has been widely used to infer the mood of customers in emails, tweets and other short communications. The base assumption is that the sentiment is a fixed value: the email is either angry or happy; positive or negative.
Hadley Wickham's testthat package has been a boon for R package authors, making it easy to write tests to verify that your code is working directly, and alerting you when you make changes to your code that inadvertently breaks things.
Secret sharing time (no, not this one): my first trip to the United States was to play pinball. I'd played a lot of pinball in my youth and I thought I was pretty good. So I booked a flight to Chicago in the mid-90's (I can't remember exactly when) to compete in the PAPA World Championships ... and promptly got my clock cleaned.