I just noticed that there is a bug in DVDAuthorGUI. Here an example:

In sup-file, if the start time is 01:15.43,02 and the end time is 01:15.44,03, DVDAuthorGui replaces the end time by 01:15.43,00 and stops with error: "ERR: sub has end (1:15:43.000)<=start (1:15:43.020), skipping".

This mystic error does not come very often, in three or four sup-files of thousends (!) files. In the other cases, I did not write down the exact values, but if I remember right, the end time has form x:xx:xx:yyy, where yyy is near to "000" (like the "020" above).