Question on Video

I Downloaded a clip off emule and saw after 10 secs it was really really pixelated.
then the same one off but was pixelated which must have meant it was a bad source or a different encode.

When a video goes pixelated due to a corrupt file isit definatly noticable or do some movie files get corrupted and you dont even notice it, can some get corrupted like maybe a little square in the bottom of the screen of pixel and would be hard to be noticed or what could cause it to be noticbly blocky?
And what causes green lines every so often?
What causes a little pixel in a corner? the same as what would cause it to be like it if it were all over the screen?

When i watch a video clip though its really pixelated compared to the other one that i swear is the same rip.
Basically i juss wanna know what would cause there to be say a slight difference to the original file like one pixel out of place?
Or if its terribly pixelated what could have caused it?

so yeh does ne1 know??