Before going any specifics some problematic apps/widgets from memory:
-Recaller
-Catorise,apmefo(basically all apps that make folders to your application screen, even making folders by hand might trigger hldon bug that will be fixed in next PR release)
-Flaslight addon

Basically all widgets that run all the time on desktop are high risk applications. Also all daemons running in background are imo in this category.

Please, if you choose to install stuff from testing&devel basic debugging skills are required . You should read bugzilla and learn to monitor your system.

.edit
also reboot is the first thing to do.