When you've had 4.2.1 for a few months, and depending on what model you have (if you have 3G or 4G, then the processor is bigger - which would eliminate crashes), the 2G or 1G would always go into safemode when booting a Cydia app. Or the apps would crash, then freeze, then quit, even the Apple ones (even ones like Music, Videos, etc). 4.0.1 doesn't need as much of a big processor as 4.2.1, so if you downgrade to that one or below on a 2G/1G, then it will hardly ever go into safemode (or reboot, etc), the only time it would go into safemode would be if you downloaded a corrupt app from Cydia.