UI Guideline: If your users aren’t expecting something to pop up when they’re moving their mouse around, try not to scare them.
Inc magazine’s “Share” button is the perfect example of what not to do. At the bottom of an article, accidentally mousing over the button will make a [useless] box pop up instantly. This should be delayed at least 2/10s of a second, more or less depending on how technical and quick your audience is with a mouse. This can be accomplished with a JavaScript delay.
The problem with the share button goes beyond that. If the button is near the bottom of my physical screen, the box will pop up over the next page link, effectively blocking me from reading the rest of the article and trying to get me to pimp their article.
But maybe that’s what Inc wanted to happen…