http://academic.csuohio.edu/polen/LC9_Help/1/14fti.htm

We can IMAGINE that the premises are true and the conclusion is false even if they are actually just the opposite. ALL invalid arguments are UNSOUND.

Consider:

      The president of the United States must be 100 years of age or older.
      George W. Bush must be 100 years of age or older.
      So, George W. Bush is president of the United States.

--or--

- If George W. Bush is president of the United States, then George W. Bush must be 100 years of age or older.
- George W. Bush must be 100 years of age or older.
- So, George W. Bush is president of the United States.

For either example, the logic is invalid and the premises are false. For the premises to be true, all of them need to be true. But, for the premises to be false, only one need be false. So, an argument with a mixture of true and false premises is still considered to be an argument with false premises--it is false that all of the premises are true. Nevertheless, in these examples, the conclusion is true.
Invalidity is a no guarantee of a true conclusion when the premises are false. False premises can lead to either a true or a false conclusion in an invalid argument. In these examples, luck rather than logic led to the true conclusion.