Occurs in level Mayelo 7 in the Festive Millas 2018 pack (https://www.lemmingsforums.net/index.php?topic=4084.0).
True physics mode switched on by holding the keyboard key (not the panel button, which toggles it) and crosshair hovering over some of the decorative teleporters to the right of the exit, which yields an exception (log file attached below).
Fixed in commit 65d61e8.