Beware, URL shorteners are being equipped with Bitcoin web mining scripts. Yes. While you are waiting for that specific link, you are actually forcing your computer to mine coins for someone else.
Where do they come from
Well, these Bitcoin web mining scripts are actually pieces of code that are often provided via companies that have affiliate programs in Bitcoin web mining. These affiliate programs allow “partners” to include code on any page on the web. It is similar to how the most online advertisement companies’ work, but instead of showing you a commercial, picture or text, the affiliate program will use your computer power to mine for Bitcoins or ALTcoins. This way, the person behind the shortened link or page will earn some online revenue.
Why are they dangerous
The moment they get dangerous is the moment that you are not aware of the fact that your device is being used for Bitcoin mining via the web. So it is very important to be aware of the fact that indeed your device(s) can be used to earn an online revenue.
The difference between advertisements and web mining scripts is the fact that web mining scripts will force your computer to solve mathematical questions. An advertisement will only do something once it has been clicked or activated (Most of the time user interaction is required).
To give you an idea of what they look like, I have collected some screenshots. Take a look.