During December 2025, Lex Leims focused on improving the robustness of data merging in the pandas-dev/pandas repository. Lex addressed an edge case in the DataFrame.merge method by ensuring that a MergeError is raised when both right_on and right_index parameters are specified, preventing ambiguous merge operations that could compromise data integrity. The solution involved targeted bug fixing and the addition of a regression test to validate the new error behavior, using Python and leveraging skills in data manipulation and testing. This work enhanced the reliability of merge semantics in pandas, reducing the risk of silent errors in data workflows and pipelines.