(I have newest version of mac and blender) I finished rendering my intro template after about 30 mins. The rectangle where it was rendering went black. I went to my downloads where I had saved it there was a quicktime file that I opened and said "converting....... QuickTime Player can't open "blender finished intro0001-0220.avi" To see if additional software is available that will enable QuickTime Player to open the movie, click Tell Me More. when I did i brought me to the apple website and said I had to download quicktime 7 even though i already have 10.6 I think. I followed the instructions and downloaded it but the file still wont open.
If an error happened before blender finished all the frames that were supposed to be rendered, I am afraid the .avi file may be corrupt. It cannot be opened or fixed. The way to render an animation the safe way is to always render as an image sequence. Meaning render each frame as a .jpg or .png file format. You would then use the VSE (Video Sequence Editor) in blender to combine all the images and save it as a .mov or .avi movie file format.
This way if some error occurs you can still restart the rendered at the point where the error occurred, instead of starting all over again because your movie file was corrupted by the incomplete rendering process.
That is how the animation industry does rendering work at a professional level too. This workflow allow for post production work, like adding visual effects and color correction after the all the frames are rendered.