The 'fatal: refusing to merge unrelated histories' error often appears when attempting to merge branches or repositories that don’t share a common commit history, which Git interprets as unrelated. This can occur when creating a new repository with existing files or trying to combine two repositories into one. To bypass this restriction, use the `--allow-unrelated-histories` flag with the `git pull` command, as in `git pull origin branch-name --allow-unrelated-histories`. This option forces Git to merge the two histories regardless of their separate origins, treating them as a single lineage moving forward. After the merge, you may need to resolve any conflicts if there are overlapping files or folders. Completing this merge with unrelated histories can simplify combining repositories but should be done carefully to maintain the project’s continuity.