Duke3D: Screen flickering on E2L11 (Lunatic Fringe)
There is a longstanding bug with overlayed Sector-Over-Sector areas where the player view is briefly transported to the wrong layer, causing screen flashes for a single frame. This can be seen in the following video at 0:16
Bisection indicates that this problem started with r7601 (commit 23d78a10) where some changes to
clipupdatesector were made.