That'd probably depend on where the bits you had yet to get where. I've managed to play avi's at 70% before - with very small chucks missing scattered aobut the file - but it's not pretty, means a lot of stuttering over missed bits. Also, the files' index is at the end of it so if the file has missing chunks at the end yet will play you might not be able to skip forward/backward due to the incomplete file index.

It really does make a difference how much is missing and where from in the file as to whether it's likely to play when incomplete. You can get utilities to fix incomplete files and make them playable - they'll obviously still have missing bits but if it's just the index for example that can be fixed easily. Hopefully that makes more sense.