We just got 7 packages in GNOME:Factory that broke because the project.diff files don't apply anymore: the packages were all updated in Factory. It's not the first time it happens, and I'm sure each time it happens, the person doesn't think it will break anything. But sometimes it does :/
Don't get me wrong: I'm really happy to see people contributing to the packages the GNOME team maintains, and especially when it comes to cleaning up things -- it makes the world a better place. But it'd be really nice to submit all updates to GNOME:Factory first. I think Mozilla:Factory was hit by this issue in the past too, and I'd guess other projects too.
So, please, think about the pain that people feel when they have to handle those manual merges, and use the devel projects.