WARNING is no good reason for stopping a flow, but it's a heads up if any other issue happens later.
CRITICAL should be the most alarming log you're ever going to receive, it should be a good excuse to wake you up at 3 am to resolve.
R. White, and J. Huang. Proceedings of the 33rd International ACM SIGIR Conference on Research and Development in Information Retrieval, page 587--594. New York, NY, USA, ACM, (2010)
A. Paranjape, R. West, L. Zia, and J. Leskovec. Proceedings of the Ninth ACM International Conference on Web Search and Data Mining, page 615--624. ACM, (2016)
M. Atzmueller, and B. Kloepper. Proc. International Conference on Intelligent Data Engineering and Automated Learning, Workshop on Methods for Interpretation of Industrial Event Logs, Heidelberg, Germany, Springer Verlag, ((accepted) 2018)