I'm working on a few projects at the same time and I often switch which mods I'm currently testing and it is very easy to forget to change them. Then you open a world and all of the sudeen all custom blocks are replaced with "bugged air". That destroys hours and days of work if you did not make a back up recently enough. Could you please add a warning when you are trying to open a world that was used with a different set of mods that your current installed ones? This would save a ton of suffering and lost work.