By now everybody should already know this. You should not rely on browser detection. User-agent sniffing is evil. Use feature detection instead. Sound and solid advice. At least until you start looking at some of the more unusual browsers.
Earlier this summer I did extensive research on smart TV and console browsers. It showed me that these browsers are a lot like mobile browsers 10 years ago — before Chrome and Safari. Everybody is trying, but nobody really knows what is right. More on that at a later time.
One important lesson I learned was that we as developers make a lot of assumptions.