When editing TimedText, it is only possible to see the result when saving, not when using "Preview changes".
When saving, the video shows up next to the text, so that you can see if it syncs or not. Since the video doesn't show up when previewing changes, there will be many, many saved versions that are just useless.
This for example:
http://commons.wikimedia.org/w/index.php?title=TimedText:Aliona_Moon_-_O_Mie_presentation_(Rom%C3%A2n%C4%83).ogv.ro.srt&action=history
Version: unspecified
Severity: enhancement