How many have experienced either or both?
Here's an old but very informative video on the subject. It shows real world examples, causes and solutions. Though the bikes used are old standard types, it's still very interesting.
http://www.youtube.com/watch?v=fvsDIq3WwVA (http://www.youtube.com/watch?v=fvsDIq3WwVA)
doing about 30 on a goped i let go of the handlebars and it wobbled hard, almost went of the handlebars!! wasn't the most stable bike, that's for sure! not too much fun either.