i will demonstrate with an example:

$ wget ftp://usernmae:password@example.com/file -c
--2011-08-08 03:16:42--  ftp://username:*password*@example.com/file
           => `file'
Resolving example.com... xxx.xxx.xxx.xxx, yyy.yyy.yyy.yyy
Connecting to example.com|xxx.xxx.xxx.xxx|:21... connected.
Logging in as username ... Logged in!
==> SYST ... done.    ==> PWD ... done.
==> TYPE I ... done.  ==> CWD not needed.
==> SIZE file ... 549807025
==> PASV ... done.    ==> REST 162791124 ... done.   
==> RETR file ... done.
Length: 387015901 (369M), 224224777 (214M) remaining (unauthoritative)

the file is around 540 megabytes, as you can see from the SIZE command. i already have around 160 megabytes downloaded from a previous session, i just want to continue now.
in the last line, it says Length is 369 megabytes. now this is really the remaining data, not the full length of the file, but wget treats it so, and calculates that what's remaining is 214 megabytes, when in fact, much more than that is remaining.

so the problem in a nutchell, is that wget should rely on info from the SIZE command to calculate length, remaining, and draw a proper progress bar.
what is happening now, is that progress bar reaches 100%, but data would still be flowing in normally till the file really ends.

thanks in advance for your time.