Here is a good one about unintended consequences and multi-functional devices. The obvious benefit of multi-functional devices is that you only have to carry around one device instead of several. So in theory, you are more likely to use a function because you are more likely to have it with you when you feel like using it.
But companies have discovered there is leverage involved in this relationship. Sometimes, people use the device just because they have it, even if they don't really have a specific desire for the function. The cliche is that when you are holding a hammer, everything looks like a nail.
Another product use insight is the risk curve. People are comfortable with a certain level of risk. So if you reduce the risk associated with a product, it does not make users safer. Instead, they use the product more aggressively or more often so that the experienced risk stays the same. This was seen in tests of non-lethal weapons given to soldiers and police. Because the person being shot was not killed, users lowered their threshold for use. In other words, they didn't need to be quite as sure an adversary deserved to be shot in order to use the weapon because they didn't have to worry that an innocent person would die. They would just be hurt/inconvenienced. This occurred with weapons such as stun guns, laser guns (that shoot bursts of intense heat that don't actually burn the skin), etc.
So here is a new proposal that I think would do wonders for public civility. A company proposed combining a taser with a cell phone. Imagine if everyone not only had a taser with them, but it was often right in their hand. Want to shut up that annoying person on the subway? Taser them. Your friend giving you a hard time across the table at the restaurant? Taser them under the table. How long would it take for us to start being really nice to each other????