Use case: trying to open native Symbian video player to play a video file.
Use QDesktopServices:

QDesktopServices::openUrl(QUrl("file:///e:/Videos/test test.mp4"));

Now, if the filename includes a space, the player will not open on Symbian Belle / Qt 4.7.4 / 111.010.1009. A blank view with title "services" is being shown.
Without a space in the filename the playback starts OK.

In Symbian Anna / Qt 4.7.3 / v025.004 both cases (with and without space) work OK.