> many 'Unexpected
> 412/416 error (Precondition Failed)'. What can cause this,
> and does it mean?
Incomplete files were updated by HTTrack, but the files 
were probably up-to-date (reget size precondition failed)

> Not only that, but lots of files i had downloaded 
previously
> were now less than 200 bytes in size, and contained a html
> page describing this error.

Weird, the redownload should be handled. Can you give more 
details ? (URL ? Specific options used ?)