A typical designer will import the plugins (for example "import QtMobility.location 1.1") and as the import works, and displays correctly on a Win32 machine, assumes all is ok.
Once it is copied across to a device and run, elements (e.g. PositionSource) using a capability that qmlviewer lacks (i.e. "Location") will cause an error.

It should be noted that the use of a plugin, with capabilities outside of "NetworkServices ReadUserData", and viewed on a Symbian device via qmlviewer.exe will either fail to work or cause qmlviewer to be unceremoniously terminated.

In the above example, qmlviewer needs to be recompiled with TARGET.CAPABILITY = NetworkServices ReadUserData Location and installed.