The difference between Mic and Line input is basically the gain. A mic will put out a very low signal and therefore require a big gain for you to record it well. Line level is higher and needs a lower gain.
In practical terms, if you attached a line-level signal to a mic input then you will get a lot of distortion. You may be able to minimise this by turning down the Mic input level the it will always be awful.
If you attach a mic to a line level input then you will get a very low signal which you will have to boost thus increasing the noise by a large degree. So only use mic inputs for mikes and line for line.
