Lines are not always recognised as they should be at three-way merge. In document A is a line that was changed in a B doc but not C. In B there was added one line and in C there were two. The packet of code are generated wrong. In B the line is considered as added and not as derivation rule as it should be. Sorry, I can't really explain and also not completly sure it actually is a bug. But better safe than sorry. There is attachment with example to explain what I am saying.