The variable binding trail for the backward chainer is stored in a simple vector but may overflow into list structure if the trail grows larger than the size of the vector: (30000). The system will continue to function normally when this happens but may slow down slightly and do more consing. (Note: We have never written a program that causes this to happen other than deliberately produced testing programs).
KnowledgeWorks and Prolog User Guide (Windows version) - 24 Mar 2017