quirky screen lock

On occasion, my screen lock fails to work, and I constantly end up accidentally dialing people or making random statuss updates and such.  When I check my screen lock settings, it is still "on".  The only way i found to remedy this is by turning off my phone and rebooting it. 

Is anyone else having this issue?  Any known causes of it?  Is there another way to remedy this (as rebooting the phone removes my dl'd ringtone)?

Thanks