This can easily happen if you install e.g. the NVidia module for the PAE kernel, which has the PAE kernel as a requirement.
It doesn’t really matter: You can boot either kernel, as long as you don’t have more modules which require compilation against a certain kernel. If you don’t like it, remove the PAE kernel and reinstall the default kernel.
Failsafe adds some options to the kernel upon boot which will allow you to boot in critical conditions, removing features known to be a possible problem (ACPI, for example). “Safe mode” in Windows.