The phrase "worst scenario" refers to a situation or outcome that is the least favorable or most undesirable. It describes the worst possible circumstances or result that could occur. Full definition
Your plan should be able to cover you and / or your family in worst scenarios. (policybazaar.com)
A slightly worse scenario happens when a sudden change in the algorithm hits a limited number of sites. (link-assistant.com)
Or, in a much worse scenario, they could do so well that they become a dreaded «invasive species» that takes over and pushes out native species. (scientificamerican.com)