A criss-cross in the branch history can cause the default merge technique to emit more conflicts than would normally be expected.
In complex merge cases, bzr merge --lca or bzr merge --weave may give better results. You may wish to bzr revert the working tree and merge again. Alternatively, use bzr remerge on particular conflicted files.
Criss-crosses occur in a branch’s history if two branches merge the same thing and then merge one another, or if two branches merge one another at the same time. They can be avoided by having each branch only merge from or into a designated central branch (a “star topology”).
Criss-crosses cause problems because of the way merge works. Bazaar’s default merge is a three-way merger; in order to merge OTHER into THIS, it must find a basis for comparison, BASE. Using BASE, it can determine whether differences between THIS and OTHER are due to one side adding lines, or from another side removing lines.
Criss-crosses mean there is no good choice for a base. Selecting the recent merge points could cause one side’s changes to be silently discarded. Selecting older merge points (which Bazaar does) mean that extra conflicts are emitted.
The weave merge type is not affected by this problem because it uses line-origin detection instead of a basis revision to determine the cause of differences.