CyanogenMod/android_frameworks_base — comment on commit 62c3efd204acdf1f7bf3bde584e2a0dfd9ca046e
File: policy/src/com/android/internal/policy/impl/LockPatternKeyguardView.java line 183
By nk222:

This leads to forced re-creation of the view on every screen timeout! So when you enter your PIN and the screen goes blank the view receives a reset and is recreated. So it never gets informed about the PIN-supply success! Then the device stays locked until SIM state changes and then displays pattern lockscreen...

behavior is described here: http://forum.xda-developers.com/showpost.php?p=22054917&postcount=434