Merging a file between SCM branches that is several thousand lines in size and has significant changes in both branches is a good way to have an unpleasant day, even if the SCM that’s being used has good support for cross-branch merging.
Yes, I know, ideally one tries to make sure that two branches don’t diverge that far but that’s not always possible, especially if there are significant changes to the design that affect the merge.
