Signal RunConfiguration::isEnabledChanged() is not necessarily connected for the active run configuration.

The following line is in ProjectExplorerPlugin::startupProjectChanged():
connect(project->activeTarget()->activeRunConfiguration(), SIGNAL(isEnabledChanged(bool)),
this, SLOT(updateRunActions()));
But the signal is not connected when the active run configuration changes for other reasons (e.g. active target changed), so the project explorer will miss updates from the new active run configuration in those cases. In the worst case, the "run" action will stay disabled for a run configuration that was initially not runnable, but has called isEnabledChanged(true) in the mean time.