The new feature of merging and un-merging errors in Rollbar allows users to combine multiple errors into a single group for streamlined management and improved metric accuracy. Users can merge errors by selecting them from the error Items feed and setting various parameters before finalizing the merge, which helps in organizing duplicated errors efficiently. If an error is incorrectly merged, it can be easily un-merged, providing flexibility in error management. The tool also includes search functionalities to view grouped or member errors separately, enhancing the debugging process. Rollbar encourages users to utilize this feature to save time and improve error handling, offering a 14-day free trial for new users to explore its benefits.