I have had iOS 6 Beta 2 on my iPhone 4S for about 3 weeks now and it wasn't until a few days ago that it began having this bug. I noticed that the timeline of when the problem began lined up perfectly with when I installed the latest official Twitter app update (4.3 if I remember correctly). One reboot gave me a message when it came back asking for permission for Twitter to access contacts and it wouldn't go away whether I denied or allowed it to. Also in the message it had a random "(null)" in the message. So I deleted the app and waited a few days to see if it would bug out again and it did not. I re-installed the app and have had no problems since. If you have the twitter app and have updated it, then that would be a great place to start since it was where I experienced the bug.