K3b doesn't continue multisession DVDs (and probably also CDs and 
BluRays). There are two ways to reproduce this bug.

1) More actions -> continue multisession project -> import session. Then 
add new file or directory, click burn. Result? All files from previous 
session have size of 0 B, and only new file has appropriate size.
2) Just click new data project, add new file or directory (without importing previous session), click burn, click misc tab, choose "continue multisession" and resulting disc has files from previous session set to size of 0 B and only new file has appropriate size.

That means K3b destroys data on discs if someone want to continue 
multisession disc.